THE

International Scientific Series

VOL. XXXV.

Frontispiece.

Sections of Igneous Rocks, illustrating the passage from the glassy to the crystalline structure.

1. Vitreous Rock. 2. Semi-Vitreous Rock. 3. Vitreous Rock with Sphærulites. 4. Rock with Crypto-crystalline Base. 5. Rock with Micro-crystalline Base. 6. Rock of Granite Structure built up entirely of Crystals.

[See pp. [63-68].

VOLCANOES
WHAT THEY ARE and WHAT THEY TEACH

BY

JOHN W. JUDD, F.R.S.

PROFESSOR OF GEOLOGY IN THE ROYAL SCHOOL OF MINES

WITH 96 ILLUSTRATIONS

SIXTH EDITION

LONDON
KEGAN PAUL, TRENCH, TRÜBNER & CO. Ltd.
PATERNOSTER HOUSE, CHARING CROSS ROAD
1903

(The rights of translation and of reproduction are reserved.)

PREFACE.

In preparing this work, I have aimed at carrying out a design suggested to me by the late Mr. Poulett Scrope, the accomplishment of which has been unfortunately delayed, longer than I could have wished, by many pressing duties.

Mr. Scrope's well-known works, 'Volcanoes' and 'The Geology and Extinct Volcanoes of Central France'—which passed through several editions in this country, and have been translated into the principal European languages—embody the results of much careful observation and acute reasoning upon the questions which the author made the study of his life. In the first of these works the phenomena of volcanic activity are described, and its causes discussed; in the second it is shown that much insight concerning these problems may be obtained by a study of the ruined and denuded relics of the volcanoes of former geological periods. The appearance of these works, in the years 1825 and 1827 respectively, did much to prepare the minds of the earlier cultivators of science for the reception of those doctrines of geological uniformity and continuity, which were shortly afterwards so ably advocated by Lyell in his 'Principles of Geology.'

Since the date of the appearance of the last editions of Scrope's works, inquiry and speculation concerning the nature and origin of volcanoes have been alike active, and many of the problems which were discussed by him, now present themselves under aspects entirely new and different from those in which he was accustomed to regard them. No one was ever more ready to welcome original views or to submit to having long-cherished principles exposed to the ordeal of free criticism than was Scrope; and few men retained to so advanced an age the power of subjecting novel theories to the test of a rigorous comparison with ascertained facts.

But this eminent geologist was not content with the devotion of his own time and energies to the advancement of his favourite science, for as increasing age and growing infirmities rendered travel and personal research impossible, he found a new source of pleasure in seeking out the younger workers in those fields of inquiry which he had so long and successfully cultivated, and in furthering their efforts by his judicious advice and kindly aid. Among the chosen disciples of this distinguished man, who will ever be regarded as one of the chief pioneers of geological thought, I had the good fortune to be numbered, and when he committed to me the task of preparing a popular exposition of the present condition of our knowledge on volcanoes, I felt that I had been greatly honoured.

In order to keep the work within the prescribed limits, and to avoid unnecessary repetitions, I have confined myself to the examination of such selected examples of volcanoes as could be shown to be really typical of all the various classes which exist upon the globe; and I have endeavoured from the study of these to deduce those general laws which appear to govern volcanic action. But it has, at the same time, been my aim to approach the question from a somewhat new standpoint, and to give an account of those investigations which have in recent times thrown so much fresh light upon the whole problem. In this way I have been led to dwell at some length upon subjects which might not at first sight appear to be germane to the question under discussion;—such as the characters of lavas revealed to us by microscopic examination; the nature and movements of the liquids enclosed in the crystals of igneous rocks; the relations of minerals occurring in some volcanic products to those found in meteorites; the nature and origin of the remarkable iron-masses found at Ovifak in Greenland; and the indications which have been discovered of analogies between the composition and dynamics of our earth and those of other members of the family of worlds to which it belongs. While not evading the discussion of theoretical questions, I have endeavoured to keep such discussions in strict subordination to that presentation of the results attained by observation and experiment, which constitutes the principal object of the work.

The woodcuts which illustrate the volume are in some cases prepared from photographs, and I am indebted to Mr. Cooper for the skill with which he has carried out my wishes concerning their reproduction. Others among the engravings are copies of sketches which I made in Italy, Hungary, Bohemia, and other volcanic districts. The whole of the wood-blocks employed by Mr. Poulett Scrope in his work on Volcanoes were placed at my disposal before his death, and such of them as were useful for my purpose I have freely employed. To Captain S. P. Oliver, R.A., I am obliged for a beautiful drawing made in the Island of Bourbon, and to Mr. Norman Lockyer and his publishers, Messrs. Macmillan & Co., for the use of several wood-blocks illustrating sun-spots and solar prominences.

J. W. J.

London: May 1881.

CONTENTS.

ILLUSTRATIONS.

VOLCANOES.

CHAPTER I.
INTRODUCTORY: NATURE OF THE INQUIRY.

'What is a volcano?' This is a familiar question, often addressed to us in our youth, which 'Catechisms of Universal Knowledge,' and similar school manuals, have taught us to reply to in some such terms as the following: 'A volcano is a burning mountain, from the summit of which issue smoke and flames.' Such a statement as this, it is probable, does not unfairly represent the ideas which are, even at the present day, popularly entertained upon the subject.

But in this, as in so many other cases, our first step towards the acquirement of scientific or exact knowledge, must be the unlearning of what we have before been led to regard as true. The description which we have quoted is not merely incomplete and inadequate as a whole, but each individual proposition of which it is made up is grossly inaccurate, and, what is worse, perversely misleading. In the first place, the action which takes place at volcanoes is not 'burning,' or combustion, and bears, indeed, no relation whatever to that well-known process. Nor are volcanoes necessarily 'mountains' at all; essentially, they are just the reverse—namely, holes in the earth's crust, or outer portion, by means of which a communication is kept up between the surface and the interior of our globe. When mountains do exist at centres of volcanic activity, they are simply the heaps of materials thrown out of these holes, and must therefore be regarded not as the causes but as the consequences of the volcanic action. Neither does this action always take place at the 'summits' of volcanic mountains, when such exist, for eruptions occur quite as frequently on their sides or at their base. That, too, which popular fancy regards as 'smoke' is really condensing steam or watery vapour, and the supposed raging 'flames' are nothing more than the glowing light of a mass of molten material reflected from these vapour clouds.

It is not difficult to understand how these false notions on the subject of volcanic action have come to be so generally prevalent. In the earlier stages of its development, the human mind is much more congenially employed in drinking in that which is marvellous than in searching for that which is true. It must be admitted, too, that the grand and striking phenomena displayed by volcanoes are especially calculated to inspire terror and to excite superstition, and such feelings most operate in preventing those close and accurate observations which alone can form the basis of scientific reasoning.

IDEAS OF THE ANCIENTS.

The ancients were acquainted only with the four or five active volcanoes in the Mediterranean area; the term 'volcano' being the name of one of these (Vulcano, or Volcano, in the Lipari Islands), which has come to be applied to all similar phenomena. It is only in comparatively modern times that it has become a known £act that many hundreds of volcanoes exist upon the globe, and are scattered over almost every part of its surface. Classical mythology appropriated Vulcano as the forge of Hephæstus, and his Roman representative Vulcan, while Etna was regarded as formed by the mountains under which a vengeful deity had buried the rebellious Typhon; it may be imagined, therefore, that any endeavour to more closely investigate the phenomena displayed at these localities would be regarded, not simply as an act of temerity, but as one of actual impiety. In mediæval times similar feelings would operate with not less force in the same direction, for the popular belief identified the subterranean fires with a place of everlasting torment; Vulcano was regarded as the place of punishment of the Arian Emperor Theodosius, while Etna was assigned to poor Anne Boleyn, the perverter of faith in the person of its stoutest defender. That such feelings of superstitious terror in connection with volcanoes are, even at the present day, far from being extinct, will be attested by every traveller who, in carrying on investigations about volcanic centres, has had to avail himself of the assistance of guides and attendants from among the common people.

Among the great writers of antiquity we find several who had so far emancipated their minds from the popular superstitions as to be able to enunciate just and rational views upon the subject of volcanoes. Until quite recent times, however, their teaching was quite forgotten or neglected, and the modern science of Vulcanology may be said to have entirely grown up within the last one hundred years.

The great pioneer in this important branch of research was the illustrious Italian naturalist Spallanzani, who, in the year 1788, visited the several volcanoes of his native land, and published an account of the numerous valuable and original observations which he had made upon them. About the same time the French geologist Dolomieu showed how much light might be thrown on the nature of volcanic action by a study of the various materials which are ejected from volcanic vents; while our own countryman. Sir William Hamilton, was engaged in a systematic study of the changes in form of volcanic mountains, and of the causes which determine their growth. At a somewhat later date the three German naturalists. Von Buch, Humboldt, and Abich, greatly extended our knowledge of volcanoes by their travels in different portions of the globe.

CHARACTER OF MODERN RESEARCHES.

The first attempt, however, to frame a satisfactory theory of volcanic action, and to show the part which volcanoes have played in the past history of our globe, together with their place in its present economy, was made in 1825, by Poulett Scrope, whose great work, 'Considerations on Volcanoes,' may be regarded as the earliest systematic treatise on Vulcanology. Since the publication of this work, many new lines of inquiry have been opened up in connection with the subject, and fresh methods of research have been devised and applied to it. More exact observations of travellers over wider areas have greatly multiplied the facts upon which we may reason and speculate, and many erroneous hypotheses which had grown up in connection with the subject have been removed by patient and critical inquiry.

We propose in the following pages to give an outline of the present state of knowledge upon the subject, and to indicate the bearings of those conclusions which have already been arrived at, upon the great questions of the history of our globe and the relations which it bears to the other portions of the universe. In attempting this task we cannot do better than take up the several lines of inquiry in the order in which they have been seized upon and worked out by the original investigators; for never, perhaps, is the development of thought in the individual mind so natural in its methods, and so permanent in its effects, as when it obeys those laws which determined its growth in the collective mind of the race. In our minds, as in our bodies, development in the individual is an epitome, or microcosmic reproduction, of evolution in the species.

CHAPTER II.
THE NATURE OF VOLCANIC ACTION.

The dose investigation of what goes on within a volcanic vent may appear at first sight to be a task beset with so many difficulties and dangers that we may be tempted to abandon it as altogether hopeless. At the first recorded eruption of Vesuvius the elder Pliny lost his life in an attempt to approach the mountain and examine the action which was taking place there; and during the last great outburst of the same volcano a band of Neapolitan students, whose curiosity was greater than their prudence, shared the same fate.

But in both these cases the inquirers paid the penalty of having adopted a wrong method. If we wish to examine the mode of working of a complicated steam-engine, it will be of little avail for us to watch the machinery when the full blast of steam is turned on, and the rapid movements of levers, pinions, and slides baffle all attempts to follow them, and render hopeless every effort to trace their connection with one another. But if some friendly hand turn off the greater part of the steam-supply, then, as the rods move slowly backwards and forwards, as the wheels make their measured revolutions, and the valves axe seen gradually opening and shutting, we may have an opportunity of determining the relations of the several parts of the machine to one another, and of arriving at just conclusions concerning the plan on which it is constructed. Nor can we doubt that the parts of the machine bear the same relation to one another, and that their movements take place in precisely the same order, when the supply of steam is large as when it is small.

Now, as we shall show in the sequel, a volcano is a kind of great natural steam-engine, and our best method of investigating its action is to watch it when a part of the steam-supply is cut off. It is true that we cannot at will control the source of supply of steam to a volcano, as we can in a steam-engine, but as some volcanoes have usually only a small steam-supply, and nearly all volcanoes vary greatly in the intensity of their action at different periods, we can, by a careful selection of the object or the time of our study, gain all those advantages which would be obtained by regulating its action for ourselves.

Spallanzani appears to have been the first to perceive the important fact, that the nature of volcanic action remains the same, however its intensity may vary. Taking advantage of the circumstance that there exists in the Mediterranean Sea a volcano—Stromboli—which for at least 2,000 years has been in a constant and regular, but not in a violent or dangerous, state of activity, he visited the spot, and made the series of careful observations which laid the foundation of our knowledge of the 'physiology of volcanoes.' Since the time of Spallanzani, many other investigators have visited the crater of Stromboli, and they have been able to confirm and extend the observations of the great Italian naturalist, as to the character of the action which is constantly taking place within it. We cannot better illustrate the nature of volcanic action than by describing what has been witnessed by numerous observers within the crater of Stromboli, where it is possible to watch the series of operations going on by the hour together, and to do so without having our judgment warped either by an excited imagination or the sense of danger.

APPEARANCE OF STROMBOLI FROM A DISTANCE.

In the sketch, [fig. 1], which was made on April 20, 1874, I have shown the appearance which this interesting volcano usually presents, when viewed from a distance. The island is of rudely circular outline, and conical form, and rises to the height of 3,090 feet above the level of the Mediterranean. From a point on the side of the mountain, masses of vapour are seen to issue, and these unite to form a cloud over the mountain, the outline of this vapour-cloud varying continually according to the hygrometric state of the atmosphere, and the direction and force of the wind. At the time when this sketch was made, the vapour-cloud was spread in a great horizontal stratum overshadowing the whole island, but it was clearly seen to be made up of a number of globular masses, each of which, as we shall hereafter see, is the product of a distinct outburst of the volcanic forces.

Viewed at night-time, Stromboli presents a far more striking and singular spectacle. The mountain, with its vapour canopy, is visible over an area having a radius of more than 100 miles. When watched from the deck of a vessel anywhere within this area, a glow of red light is seen to make its appearance from time to time above the summit of the mountain; this glow of light may be observed to increase gradually in intensity, and then as gradually to die away. After a short interval the same appearances are repeated, and this goes on till the increasing light of the dawn causes the phenomenon to be no longer visible. The resemblance presented by Stromboli to a 'flashing light' on a most gigantic scale is very striking, and the mountain has long been known as 'the lighthouse of the Mediterranean.'

It must be pointed out, however, that in two very important particulars the appearances presented by Stromboli differ markedly from those rhythmical gleams exhibited by the 'flashing-lights' of our coasts. In the first place, the intervals between successive flashes are very unequal, varying from less than one minute to twenty minutes, or even more; and in the second place, the duration and intensity of the red glow above the mountain are subject to like variation, being sometimes a momentary scarcely visible gleam, and at others a vivid burst of light which illuminates the sky to a considerable distance round.

Fig. 1.—Stromboli, viewed from the North-west, April 1874.

Fig. 2.—Map of tub Island of Stromboli. (Scale about two inches to a mile.)

GENERAL FEATURES OF THE MOUNTAIN.

Let us now draw near and examine this wonderful phenomenon of a mountain which seemingly ever burns with fire, and yet is not consumed. The general form of the Island of Stromboli will be gathered from an inspection of the plan, [fig. 2], which is copied from a map published by the Italian Government. When we land upon the island, we find that it is entirely built up of such materials as we know to be ejected from volcanoes; indeed, it resembles on a gigantic scale the surroundings of an iron furnace, with its heaps of cinders and masses of slag. The irregularity in the form of the island is at once seen to be due to the action of the wind, the rain, and the waves of the surrounding sea, which have removed the loose, cindery materials at some points, and left the hard, slaggy masses standing up prominently at others.

This great heap of cindery and slaggy materials rises, as we have said, to a height of more than 3,000 feet above the sea-level, but even this measurement does not give a just idea of its vast bulk. Soundings in the sea surrounding the island show that the bottom gradually shelves around the shores to the depth of nearly 600 fathoms, so that Stromboli is a great conical mass of cinders and slaggy materials, having a height of over 6,000 feet, and a base whose diameter exceeds four miles.

The general form and proportions of this mass will be better understood by an examination of the section, [fig. 3], which is also constructed from the materials furnished by the map of the island issued by the Italian Government. The same section, and the map, [fig. 2], will serve to make clear the position and relations of the point on the mountain at which the volcanic activity takes place. At a spot on the north-west slope of the mountain, about 1,000 feet below its summit, and 2,000 feet above the level of the sea, there exists a circular depression, the present active 'crater' of the volcano; and leading down from this to the sea there is a flat slope making an angle of about 35° with the horizon, and known as the 'Sciarra.' The Sciarra is bounded by steep cliffs, as shown in the sketch [fig. 1], and the plan [fig. 2].

Fig. 3.—Section through the Island of Stromboli from n.w. to s.e.

a. Highest summit of the mountain, c. Cratère del Fossa, b. Point overlooking the crater, d. Steep slope known as the Sciarra del Fuoco. e. Continuation of the same slope beneath the level of the sea. f. Steep cliffs of the Punta dell' Omo.

FORM AND FUNCTION OF THE CRATER.

If we climb up to this scene of volcanic activity, we shall be able to watch narrowly the operations which are going on there. On the morning of the 24th of April, 1874, I paid a visit to this interesting spot in order to get a near view of what was taking place. On reaching a point upon the side of the Sciarra, from which the crater was in full view before me, I witnessed, and made a sketch of, an outburst which then took place, and this sketch has been reproduced in [fig. 4]. Before the outburst, numerous light curling wreaths of vapour were seen ascending from fissures on the sides and bottom of the crater. Suddenly, and without the slightest warning, a sound was heard like that produced when a locomotive blows off its steam at a railway-station; a great volume of watery vapour was at the same time thrown violently into the atmosphere, and with it there were hurled upwards a number of dark fragments, which rose to the height of 400 or 500 feet above the crater, describing curves in their course, and then falling back upon the mountain. Most of these fragments tumbled into the crater with a loud, rattling noise, but some of them fell outside the crater, and a few rolled down the steep slope of the Sciarra into the sea. Some of these falling fragments were found to be still hot and glowing, and in a semi-molten condition, so that they readily received the impression of a coin thrust into them.

Fig. 4.—The Crates of Stromboli as viewed from the side of the Sciarra during an eruption on the morning of April 24, 1874.

APERTURES AT THE BOTTOM OF THE CRATER.

But on the upper side of the crater, at the point marked 6, on the section [fig. 3], there exists a spot from which we can look down upon the bottom of the crater, and view the operations taking place there. This is the place where Spallanzani and other later investigators have carried on their observations, and, when the wind is blowing from the spectator towards the crater, he may sit for hours watching the wonderful scene displayed before him. The black slaggy bottom of the crater is seen to be traversed by many fissures or cracks, from most of which curling jets of vapour issue quietly, and gradually mingle with and disappear in the atmosphere. But besides these smaller cracks at the bottom of the crater, several larger openings are seen, which vary in number and position at different periods; sometimes only one of these apertures is visible, at others as many as six or seven, and the phenomena presented at these larger apertures are especially worthy of careful investigation.

These larger apertures, if we study the nature of the action taking place at them, may be divided into three classes. From those of the first class, steam is emitted with loud, snorting puffs, like those produced by a locomotive-engine, but far less regular and rhythmical in their succession. In the second class of apertures masses of molten material are seen welling out, and, if the position of the aperture be favourable, flowing outside the crater; from this liquid molten mass steam is seen to escape, sometimes in considerable quantities. The openings of the third class present still more interesting appearances. Within the walls of the aperture a viscid or semi-liquid substance is seen slowly heaving up and down. As we watch the seething mass the agitation within it is observed to increase gradually, and at last a gigantic bubble is formed which violently bursts, when a great rush of steam takes place, carrying fragments of the scum-like surface of the liquid high into the atmosphere.

If we visit the crater by night, the appearances presented are found to be still more striking and suggestive. The smaller cracks and larger openings glow with a ruddy light. The liquid matter is seen to be red- or even white-hot, while the scum or crust which forms upon it is of a dull red colour. Every time a bubble bursts and the crust is broken up by the escape of steam, a fresh, glowing surface of the incandescent material is exposed. If at these moments we look up at the vapour-cloud covering the mountain, we shall at once understand the cause of the singular appearances presented by Stromboli when viewed from a distance at night, for the great masses of vapour are seen to be lit up with a vivid, ruddy glow, like that produced when an engine-driver opens the door of the furnace and illuminates the stream of vapour issuing from the funnel of his locomotive.

Let us now endeavour to analyse the phenomena so admirably displayed before us in the crater of Stromboli. The three essential conditions on which the production of these phenomena seems to depend are the following: first, the existence of certain apertures or cracks communicating between the interior and the surface of the earth; secondly, the presence of matter in a highly heated condition beneath the surface; and thirdly, the existence of great quantities of water imprisoned in the subterranean regions—which water, escaping as steam, gives rise to all those active phenomena we have been describing.

CAUSE OF THE GLOWING LIGHT.

We have said, at the outset, that there exists no analogy whatever between the action which takes place in volcanoes and the operation of burning or combustion. Occasionally, it is true, certain inflammable substances are formed by the action going on within the volcano, and these inflammable substances, taking fire, produce real flames. Such flames are, however, in almost all cases only feebly luminous, and do not give rise to any conspicuous appearances. What is usually taken for flame during volcanic eruptions is simply, as we have already pointed out, the glowing red-hot surface of a mass of molten rock, reflected from a vapour-cloud hanging over it. The red glow observed over a volcano in eruption is indeed precisely similar in its nature and origin to that which is seen above London during a night of heavy fog, and which is produced by the reflection of the gas-lights of the city from the innumerable particles of water-vapour diffused through the atmosphere. Fires, of course, occur when the molten and incandescent materials poured out from a volcano come in contact with inflammable substances, such as forests and houses, but in these cases the combustion is quite a secondary phenomenon.

There is another popular delusion concerning volcanic action, which it may be necessary to refer to and to combat. From the well-known fact that sulphur or brimstone is found abundantly in volcanic regions, the popular belief has arisen that this highly inflammable substance has something to do with the production of the eruptions of volcanoes. In school-books which were, until comparatively recent years, in constant use in this country, the statement may be found that by burying certain quantities of sulphur, iron-pyrites, and charcoal in a hole in the ground, we may form a miniature volcano, and produce all the essential phenomena of a volcanic eruption. No greater mistake could possibly be made. The chemical reactions which take place when sulphur and other substances are made to act upon each other differ entirely from the phenomena of volcanic action. The sulphur which is found in volcanic regions is the result and not the cause of volcanic action. Among the most common substances emitted from volcanic vents along with the steam are the two gases, sulphurous acid and sulphuretted hydrogen. When these two gases come into contact with one another, chemical action takes place, and the elements contained in them—oxygen, hydrogen, and sulphur—are free to group themselves together in an entirely new fashion; the consequence of this is that water and sulphuric acid (oil of vitriol) are formed, and a certain quantity of sulphur is set free. The water escapes into the atmosphere, the sulphuric acid combines with lime, iron, or other substances contained in the surrounding rocks, and the sulphur builds up crystals in any cavities which may happen to exist in these rocks.

VOLCANIC ACTION RESEMBLES BOILING.

If, however, careful and exact observations, like those carried on at Stromboli, compel us to reject the popular notions concerning the supposed resemblance between volcanic action and the combustion of sulphur or other substances, they nevertheless suggest analogies with certain other simple and well-known operations. And in pursuing these analogies, we are led to the recognition of some admirable illustrations both of the attendant phenomena and of the true cause of volcanic outbursts.

No one can look down on the mass of seething material in violent agitation within the fissures at the bottom of the crater of Stromboli, without being forcibly reminded of the appearances presented by liquids in a state of boiling or ebullition. The glowing material seems to be agitated by two kinds of movements, the one whirling or rotatory, the other vertical or up-and-down in its direction. The fluid mass in this way appears to be gradually impelled upwards, till it approaches the lips of the aperture, when vast bubbles are formed upon its surface, and to the sudden bursting of these the phenomena of the eruption are due.

Now if we take a tall narrow vessel and fill it with porridge or some similar substance of imperfect fluidity, we shall be able, by placing it over a fire, to imitate very closely indeed the appearances presented in the crater of Stromboli. As the temperature of the mass rises, steam is generated within it, and in the efforts of this steam to escape, the substance is set in violent movement. These movements of the mass are partly rotatory and partly vertical in their direction; as fresh steam is generated in the mass its surface is gradually raised, while an escape of the steam is immediately followed by a fall of the surface. Thus an up-and-down movement of the liquid is maintained, but as the generation of steam goes on faster than it can escape through the viscid mass, there is a constant tendency in the latter to rise towards the mouth of the vessel. At last, as we know, if heat continues to be applied to the vessel, the fluid contents will be forced up to its edge and a catastrophe will occur; the steam being suddenly and violently liberated from the bubbles formed on the surface of the mass, and a considerable quantity of the material forcibly expelled from the vessel. The suddenness and violence of this catastrophe is easily accounted for, if we bear in mind that the escaping steam acts after the manner of a compressed spring which is suddenly released. Steam is first formed at the bottom of the vessel which is in contact with the fire; but here it is under the pressure of the whole mass of the liquid, and moreover, the viscidity of the substance tends to retard the union of the steam bubbles and their rise to the surface of the mass. But when the pressure is relieved by the bursting of bubbles at the surface, the whole of the generated steam tends to escape suddenly.

ESCAPE OF STEAM-BUBBLES FROM LAVA.

Now within the crater of Stromboli we have precisely the necessary conditions for the display of the same series of operations. In the apertures at the bottom there exists a quantity of imperfectly fluid materials at a higher temperature, containing water entangled in its mass. As this water passes into the state of steam it tends to escape, and in so doing puts the whole mass into violent movement. When the steam rises to the surface, bubbles are formed, and the formation of these bubbles is promoted by the circumstance that the liquid mass, where exposed to the atmosphere, becomes chilled, and thereby rendered less perfectly fluid. By the bursting of these bubbles the pressure is partially relieved, and a violent escape of the pent-up steam takes place through the whole mass. Equilibrium being thus restored, there follows a longer or shorter interval of quiescence, during which steam is being generated and collected within the mass, and the series of operations which we have described then recommences.

There is one other consideration which must be borne in mind in connection with this subject. It is well known that if water be subjected to sufficiently great pressure it may be raised to a very high temperature and still retain its liquid condition. When this pressure is removed, however, the whole mass passes at once into the condition of steam or water-gas; and the gas thus formed at high temperatures has a proportionably high tension. In a Papin's digester water confined in a strong vessel is raised to temperatures far above its ordinary boiling-point, and from any opening in such a vessel the steam escapes with prodigious violence. Now, at considerable depths beneath the earth's surface, and under the pressure of many hundreds or thousands of feet of solid rock, water still retaining its liquid condition may become intensely heated. When the pressure is relieved by the formation of a crack or fissure in the superincumbent mass of rock, the escape of the superheated steam will be of very violent character, and may be attended with the most striking and destructive results. In the existence of high temperatures beneath the earth's surface, and the presence in the same regions of imprisoned water capable of passing into the highly elastic gas which we call steam, we have a cause fully competent to produce all the phenomena which we have described as occurring at Stromboli.

It may at first sight appear that the grand and terrible displays of violence witnessed during a great volcanic eruption differ fundamentally in their character and their origin from those feeble outbursts which we are able to examine closely and analyse rigorously at Stromboli. But that such is not the case a few simple considerations will soon convince us.

STROMBOLI COMPARED WITH VESUVIUS.

Although Stromboli usually displays the subdued and moderate activity which we have been describing, yet the intensity of the action going on within it is subject to considerable variation. Occasionally the violence of the outbursts is greatly increased—the roaring of the steam-jets may be heard for many miles around, considerable streams of incandescent liquefied rock flow down the Sciarra into the sea, and the explosions in the crater are far more frequent and energetic, cinders and fragments of rock being scattered all over the island and the surrounding seas.

On the other hand, volcanoes like Vesuvius, which are sometimes the scene of eruptions on the very grandest scale, at others subside into a temporary state of moderate activity quite similar in character to that which is the normal condition of Stromboli. Thus, shortly before the great eruption of Vesuvius in April 1872, a small cone was formed near the edge of the crater, and during some months observers could watch, in ease and safety, a series of small explosions taking place, quite similar in their character and attendant phenomena to those which we have described as occurring at Stromboli. French geologists are in the habit of defining the condition of activity in a volcano by speaking of the more quiet and, regular state as the 'Strombolian stage,' and the more violent and paroxysmal as the 'Vesuvian stage'; but the two conditions are, as we have seen, presented by the same volcano at different periods, and pass into one another by the most insensible gradations.

We must now proceed to compare the grand and terrible appearances presented during a great eruption with those more feeble displays which we have been describing, to show that in all their essential features these different kinds of outbursts are identical with one another, and must be referred to the action of similar causes.

The volcanic eruption which has been most carefully studied in recent times is that which we have already referred to as occurring at Vesuvius, in the month of April 1872. With the exception, perhaps, of that which took place in October 1822, this eruption was the grandest which has broken out at Vesuvius during the present century. Owing to the circumstance of its proximity to the great city of Naples, Vesuvius has always been the most carefully watched of all volcanoes, and in recent years the erection of an observatory, provided with instruments for recording the smallest subterranean tremors affecting the mountain, has facilitated the carrying on of those continuous and minute observations which are so necessary for exact scientific inquiry.

Fig 5. Vesuvius in Eruption, as seen from Naples, April 26, 1872. (From a photograph)

VESUVIUS ERUPTION OF 1872.

On the occasion of this outburst, the aid of instantaneous photography was first made available for obtaining a permanent record of the appearances displayed at volcanic eruptions. In [fig. 5] we have one of these photographs, which was taken at 5 o'clock P.M. on April 26, 1872, transferred to a wood-block and engraved. In examining it we feel sure that we are not being misled by any exaggeration or error on the part of the artist. Vesuvius rises to the height of nearly 4,000 feet above the level of the sea, and an inspection of the photograph proves that the vapours and rock-fragments were thrown to the enormous height of 20,000 feet, or nearly four miles, into the atmosphere.

The main features of this terrifying outburst were as follows. For more than a twelvemonth before, the activity of the forces at work within the mountain appeared to be gradually increasing, and the great eruption commenced on April 24, attained its climax on the 26th, and began to die out on the following day. During the eruption the bottom of the crater was entirely broken up, and the sides of the mountain were rent by fissures in all directions. So numerous were these fissures and cracks that liquid matter appeared to be oozing from every part of its surface, and, as Professor Palmieri, who witnessed the outburst from the observatory, expressed it, 'Vesuvius sweated fire.' One of the fissures was of enormous size, extending from the summit to far beyond the base of the cone; the scar left by this gigantic rent being plainly visible at the present day.

From the great opening or crater at the summit, and from some of the fissures on the sides of the mountain, enormous volumes of steam rushed out with a prodigious roaring sound, the noise being so terrific that the inhabitants of Naples, five miles off, fled from their houses and spent the night in the open streets. Although this roaring sound appeared at a distance to be continuous, yet those upon the mountain could perceive that it was produced by detonations or explosions rapidly following one another. Each of these explosions was accompanied by the formation of a great globe of white vapour, which, rising into the atmosphere, swelled the bulk of the vast cloud overhanging the mountain. An inspection of the photographs (see [fig. 5]) shows that the great vapour-cloud over Vesuvius was made up of the globular masses ejected at successive explosions. Each of these explosive upward rushes of steam carried along with it a considerable quantity of solid fragments, and these fell in great numbers all over the surface of the mountain, breaking the windows of the observatory, and making it dangerous to be out of doors.

We have said that lava, or molten rock, appeared to be issuing from the very numerous cracks formed all over the flanks of the mountain. But at three points this molten rock issued in such quantities as to form great, fiery floods, which rushed down the sides of the mountain, and flowed to a considerable distance beyond its base. The largest of these lava-floods overwhelmed and destroyed the two villages of Massa di Somma and San Sebastiano, besides many country houses in the neighbourhood.

STEAM EMITTED FROM LAVA-CURRENT.

A very marked and interesting feature exhibited by these three lava-floods was the quantity of watery vapour which they gave off during their flow. All along their course, enormous volumes of steam were evolved from them, as will be seen by an inspection of the photograph. Indeed, such was the abundance and tension of the steam thus escaping from the surfaces of the lava-currents that it forced the congealing rock up into great bubbles and blisters, and gave rise to the formation of innumerable miniature volcanoes, varying in size from a beehive to a cottage, some of which remained in a state of independent activity for a considerable time.

So far, what we have described as taking place at Vesuvius, in April 1872, has been only the repetition on a £Eur grander scale of the three kinds of action which we have shown to be constantly taking place at Stromboli; namely, the formation of cracks or fissures in the earth's surface, the escape of steam with explosive violence from these openings, often propelling rock-fragments into the atmosphere, and the outwelling, under the influence of this compressed steam, of masses of molten materials.

There were some other appearances presented at the great outburst at Vesuvius, which do not seem at first sight to find any analogies in the manifestations of the more feeble action continually going on at Stromboli.

Before and during the great outbreak of April 1872, Vesuvius itself and the whole country round were visited with earthquake-shocks, or tremblings of the ground. The sensitive instruments in the Vesuvian Observatory showed the mountain daring the eruption to be in a constant state of tremor. These earthquakes are not, as is commonly supposed, actual upheavings of the earth's surface, but are vibrations propagated through the solid materials of which the earth is built up. We cannot stamp our feet upon the ground without giving rise to such vibrations, though our senses may not be sufficiently acute to perceive them. The explosive escape of steam from a crack is a cause sufficiently powerful to produce a shock which is propagated and may be felt for a considerable distance round. Even on Stromboli an observer at the edge of the crater may notice that each explosive outburst of steam is accompanied by a perceptible tremor of the ground, and in the case of Vesuvius the violent shocks produced by the escape of far larger volumes of steam give rise to proportionately stronger vibrations. The nature and origin of those far more terrible and destructive shocks which sometimes accompany, and more frequently precede, great volcanic eruptions, we shall consider in the sequel.

CAUSE OF LIGHTNING DURING ERUPTIONS.

Another striking phenomenon which was exhibited in the great eruption of Vesuvius in 1872 was the vivid display of lightning accompanied by thunder. The uprushing current of steam and rock-fragments forms a vertical column, but as the steam condenses it spreads out into a great horizontal cloud which is seen to be made up of the great globes of vapour emitted at successive explosions. When there is little or no wind the vertical column with a horizontal cloud above it bears a striking resemblance to the stone-pine trees which form so conspicuous a feature in every Neapolitan landscape. Around this column of vapour the most vivid lightning constantly plays and adds not a little to the grand and awful character of the spectacle of a volcanic eruption, especially when it is viewed by night.

In the eruption of 1872 a strong wind blowing from the north-west destroyed the usual regular appearance of this 'pine-tree appendage' to the mountain, which is so well known to, and dreaded by the inhabitants of Naples; the cloud, as will be seen from the photograph ([fig. 5], facing p. 24), was blown on one side, and most of the falling fragments took the same direction.

It is well known that when high-pressure steam IS allowed to escape through an orifice, electricity is abundantly generated by the friction, and Sir William Armstrong's hydro-electric machine is constructed on this principle. Every volcano in violent eruption is a very efficient hydro-electric machine, and the uprushing column is in a condition of intense electrical excitation. This result is probably aided by the friction of the solid particles as they are propelled upwards and fall back into the crater. The restoration of the condition of electrical stability between this column and the surrounding atmosphere is attended with the production of frequent lightning-flashes and thunder-claps, the found of the latter being usually, however, drowned in the still louder roar of the uprushing steam-column.

The discharge of Buch large quantities of steam into the atmosphere soon causes the latter to be saturated with watery vapour, and there follows an excessive rainfall; long-continued rain and floods were an accompaniment of the great Vesuvian outbreak of 1872, as they have been of almost all great volcanic eruptions. The Italians, indeed, dread the floods which follow an eruption more than the fiery streams of lava which accompany it—for they have found the mud-streams (lave di fango), formed by rain-water sweeping along the loose volcanic materials, to be more widely destructive in their effects than the currents of molten rock (lave di fuoco).

Besides the phenomena which we have now described as accompanying a great volcanic outburst, many others have undoubtedly been recorded by apparently trustworthy authorities. But, in dealing with the descriptions of these grand and terrible events, we must always be on our guard against accepting as literal facts, the statements made by witnesses, often writing at some distance from the scene of action, and almost always under the influence of violent excitement and terror. The desire to administer to the universal love of the marvellous, and the tendency to exaggeration, will usually account for many of the wonderful statements contained in such records; and, even where the witness is accurately relating events which he thinks passed before his eyes, we must remember that it is probable he may have had neither the opportunity nor the capacity for exact observation.

The more carefully we sift the accounts which have been preserved of great volcanic outbursts, the more are we struck by the fact that the appearances described can be resolved into a few simple operations, the true character of which has been distorted or disguised by the want of accurate observation on the part of the witnesses.

SIMILARITY OF FEEBLE AND VIOLENT ERUPTIONS.

We are thus led to the conclusion that the grand and terrible appearances displayed at Vesuvius and other volcanoes in a state of violent eruption do not differ in any essential respect from the phenomena which we have witnessed accompanying the miniature outbursts of Stromboli. And we are convinced, by the same considerations, that the forces which give rise to the feeble displays in the latter case would produce, if acting with greater intensity and violence, all the magnificent spectacles presented in the former.

In Vesuvius and Stromboli alike, the active cause of all the phenomena exhibited is found to be the escape of steam from the midst of masses of incandescent liquefied rock. The violence, and therefore the grandeur and destructive effects of an eruption, depend upon the abundance and tension of this escaping steam.

There is one respect in which volcanic phenomena are especially calculated to excite the fear and wonder of beholders—namely, in the sudden and apparently spontaneous character of their manifestations. Eclipses were regarded as equally portentous with volcanic eruptions till astronomers learned not only to explain the causes which gave rise to them, but even to predict to the second the times of their occurrence. If we were able in like manner to warn the inhabitants of volcanic regions of the approach of a grand eruption, the fear and superstition with which these events are now regarded would doubtless be in great part dispelled. The power of prediction is alike the crucial test and the crowning triumph of a scientific theory.

But, although natural philosophers are able to assign the causes to which the grand operations of volcanoes are due, and also to explain all the varied appearances which accompany them, they have not as yet so far mastered the laws which govern volcanic action as to be able to predict the periods of their manifestation.

That these operations, like all others going on upon the globe, are governed by great natural laws we cannot for a moment doubt. And that, in all probability, more careful and exact observation and reasoning will at some future time lead us to the recognition of these laws, every student of nature is sanguine. But at the present time, it must be confessed, we are very far indeed from being able to afford that crowning proof of the truth of our theories of volcanic action which is implied in the power of predicting the period and degree of intensity of their manifestations.

ERUPTIONS AND THE INTERVALS BETWEEN THEM.

There are, however, some observations which lead us to hope that the time may not be far distant when we shall have so £Eur obtained a knowledge of the conditions on which volcanic action depends as to be able to form some judgment as to its manifestations in the future at any particular locality. But we must recollect that these conditions axe very numerous and complicated, and that some of them may lie almost entirely outside our sphere of observation; hence hasty attempts in this direction, such as have recently been made, are to be deprecated by every true lover of science.

Concerning the eruptions that have taken place at those volcanic centres which have been known from a remote antiquity, we have records from which we can determine the intervals separating these outbursts and their relative violence. A critical examination of these records leads to the following conclusions:—

(1.) A long period of quiescence is generally followed by an eruption which is either of long duration or of great violence.

(2.) A long-continued, or very violent eruption is usually followed by a prolonged period of repose.

(3.) Feeble and short eruptions usually succeed one another at brief intervals.

(4.) As a general rule, the violence of a great eruption is inversely proportional to its duration.

It will be seen that these general conclusions are in perfect harmony with the theory that volcanic outbursts are due to the accumulation of steam at volcanic centres, and that the tension of this imprisoned gas eventually overcomes the repressing forces which tend to prevent its manifestation. Before astronomers had learnt to determine all the conditions on which the production of eclipses depends, they had found that these phenomena succeed one another at regular intervals. The discovery of such astronomical cycles was a great advance in our knowledge of the heavenly bodies, and in the same way the determination of these general relations between the intensity and duration of volcanic outbursts and the intervals of time which separate them may be regarded as the first step towards the discovery of the laws which govern volcanic activity.

In the actual determination of the conditions upon which the occurrence of volcanic eruptions depends, it must be confessed, however, that very little has as yet been done. This is in part due to the fact that some at least of these conditions lie beyond the limits of direct observation. But it must also be admitted, on the other hand, that little has been as yet accomplished towards the careful and systematic observation of those phenomena which may, and probably do, exert an influence in bringing about volcanic outbursts.

INFLUENCE OF ATMOSPHERIC CONDITIONS.

In the Lipari Islands there has prevailed a belief, from the very earliest period of history, that the feeble eruptions of Stromboli are in some way dependent upon the condition of the atmosphere. These islands were known to the ancients as the Æolian Isles, from the fact that they were once ruled over by a king of the name of Æolus. It seems not improbable that Æolus was gifted with natural powers of observation and reasoning far in advance of those of his contemporaries. A careful study of the vapour-cloud which covers Stromboli would certainly afford him information concerning the hygrometric condition of the atmosphere; the form and position assumed by this vapour-cloud would be a no less perfect index of the direction and force of the wind; and, if the popular belief be well founded, the frequence and violence of the explosions taking place from the crater would indicate the barometric pressure. From these data an acute observer would be able to issue 'storm-warnings' and weather-prognostics of considerable value. In the vulgar mind, the idea of the prediction of natural events is closely bound up with that of their production; and the shrewd weather-prophet of Lipari was after his death raised to the rank of a god, and invested with the sovereignty of the winds.

Whether the popular idea that the outbursts of Stromboli are regulated by atmospheric conditions has any foundation is still open to grave doubt. It seems to be certain, however, that during autumn and winter the more violent paroxysms of the volcano occur, and that in summer the action which takes place is far more regular and equable. It would be of the greatest benefit to science if an observatory were erected beside the crater of Stromboli, where a careful record might be kept of all atmospheric changes, and of the synchronous manifestations of the volcanic forces.

A little consideration will show that it is a by no means unreasonable supposition that variations in atmospheric pressure may exercise a very important influence in bringing about volcanic outbursts. Changes in the barometer to the extent of two inches within a very short period are not uncommon occurrences. A very simple calculation will show that the fall of the mercury in the barometer to the extent of two inches indicates the removal of a weight of two millions of tons from each square mile of the earth's surface where this change takes place. Now, if we suppose, as we have good ground for doing, that under volcanic areas vast quantities of superheated water are only prevented from flashing into steam by the superincumbent pressure, a relief of this pressure to the extent of two millions of tons on every square mile could scarcely fail to produce very marked effects. The way in which explosions in fiery coal-mines generally follow closely upon sudden falls in the atmospheric pressure is now well known; and coal-mine explosions and volcanic outbursts have this in common, that both result from the sudden and violent liberation of subterranean gases. There are not a few apparently well-authenticated accounts of volcanic and earthquake phenomena following closely on peculiar atmospheric conditions, and the whole question of the relation of the volcanic forces to atmospheric pressure, as Spallanzani himself so long ago pointed out, is deserving of a most careful and rigorous investigation.

SUPPOSED TIDAL EFFECTS.

There is one other consideration which has frequently been urged as worthy of especial attention, in dealing with the question of the exciting causes of volcanic outbursts. If volcanoes were, as was at one time almost universally supposed, in direct communication with a great central mass of liquefied materials, or even if any large reservoirs of such liquids existed beneath volcanic districts, as others have imagined, then the different mobility of the solid and liquid portions of the earth's mass would give rise to tidal effects similar to those occurring in the surface waters of the globe. Under such circumstances, volcanic outbursts, like the tides, would be determined by the relative positions of the sun and moon to our globe. It is certain, however, that no very direct relation has yet been established between the lunar periods and those of volcanic outbursts, though recent close observations upon the crater of Vesuvius, by Professor Palmieri, do seem to lend support to the view that such relations may exist.

At the present time, therefore, it must be admitted that vulcanologists have only just commenced those series of exact and continuous observations which are necessary to determine the conditions that regulate the appearance of volcanic phenomena. The study of the laws of volcanic action is yet in its infancy. But the establishment of observatories on Vesuvius and Etna 18 fall of promise for the future, and when we consider the advances which have been made, during the last one hundred years, in our knowledge of the true nature of volcanic action, we need not despair that the extension of the same methods of inquiry will lead to equally important results concerning the conditions which determine and the laws which govern it.

In the meanwhile, it is no small gain to have established the fact that volcanic phenomena, divested of all those wonderful attributes with which superstition and the love of the marvellous have surrounded them, are operations of nature obeying definite laws, which laws we may hope by careful observation and accurate reasoning to determine; and that the varied appearances, presented alike in the grandest and feeblest outbursts, can all be referred to one simple cause—namely, the escape, from the midst of masses of molten materials, of imprisoned steam or water-gas.

CHAPTER III.
THE PRODUCTS OF VOLCANIC ACTION.

While Spallanzani was engaged in investigating the nature of the action going on at Stromboli and other Italian volcanoes, his contemporary Dolomieu was laying the foundation of another important branch of vulcanology by studying the characters of the different materials of which volcanoes are built up. Since the publication of Dolomieu's admirable works on the rocks of the Lipari and Ponza Islands, science has advanced with prodigious strides. The chemist has taught us how to split up a rock into its constituent elements and to determine the proportions of these to one another with mathematical precision; the mineralogist has done much in the investigation of the characters and mode of origin of the crystalline minerals which occur in these rocks; and the microscopist has shown how the minute internal structure of these rocks may be made clearly manifest. We shall proceed to give a sketch of the present state of knowledge obtained by these different kinds of investigations, concerning the materials which are ejected from volcanic vents.

The most abundant of the substances which are ejected from volcanoes is steam or water-gas, which, as we have seen, issues in prodigious quantities during every eruption. But with the steam a great number of other volatile materials frequently make their appearance. The chief among these are the add gases known as hydrochloric acid, sulphurous acid, sulphuretted hydrogen, carbonic add, and boracic acid; and with these acid gases there issue hydrogen, nitrogen, ammonia, the volatile metals arsenic, antimony, and mercury, and some other substances. In considering the nature of the products which issue from volcanic fissures, it must be remembered that many substances which under ordinary circumstances do not exhibit marked volatility are nevertheless easily carried away in fine particles when a current of steam is passed over them. As we shall have to point out in the sequel, different volatile substances have a tendency to make their appearance at volcanic vents according as the intensity of the action going on within it varies.

The volatile substances issuing from volcanic fissures at high temperatures react upon one another, and many new compounds are thus formed. We have already seen how, by the action of sulphurous acid and sulphuretted hydrogen on each other, the sulphur so common in volcanic districts has been separated and deposited. The hydrochloric acid acts very energetically on the rocks around the vents, uniting with the iron in them to form the yellow ferric-chloride. The rocks all round a volcanic vent are not unfrequently found coated with this yellow substance, which is almost always mistaken by casual observers for sulphur. In many volcanoes the constant passage through the rocks of the various acid gases has caused nearly the whole of the iron, lime, and alkaline materials of the rocks to be converted into soluble compounds known as sulphates, chlorides, carbonates, and borates; and, on the removal of these by the rain, there remains a white, powdery substance, resembling chalk in outward appearance, but composed of almost pure silica. There are certain cases in which travellers have visited volcanic islands where chemical action of this kind has gone on to such an extent, that they have been led to describe the islands as composed entirely of chalk.

GASES EMITTED FROM VOLCANOES.

Some of the substances issuing from volcanic vents, such as hydrogen and sulphuretted hydrogen, are inflammable, and when they issue at a high temperature, these gases burst into flame the moment that they come into contact with the air. Hence, when volcanic fissures axe watched at night, faint lambent flames are frequently seen playing over them, and sometimes these flames are brilliantly coloured, through the presence of small quantities of certain metallic oxides. Such volcanic flames, however, are scarcely ever strongly luminous and, as we have already seen, the red, glowing light which is observed over volcanic mountains in eruption is due to quite another cause. The study by the aid of the spectroscope of the flames which issue from volcanic vents promises to throw much new light on the rarer materials ejected by volcanoes. Spectroscopic observations of this kind have already been commenced by Janssen, at Stromboli and Santorin.

Some of the volatile substances issuing from volcanic vents, are at once deposited when they come in contact with the cool atmosphere, others form new compounds with one another and the constituents of the atmosphere, while others again attack the materials of the surrounding rocks and form fresh chemical compounds with some of their ingredients. Thus, there are continually accumulating on the sides and lips of volcanic fissures deposits of sulphates, chlorides, and borates of the alkalies and alkaline earths, with sal-ammoniac, sulphur, and the oxides and sulphides of certain metals. The lips of the fissures from which steam and acid gases issue in volcanoes are constantly seen to be coated with yellow and reddish-brown incrustations, consisting of mixtures, in varying proportions, of these different materials, and these sometimes assume the form of stalactites and pendent masses.

DEPOSITS AROUND VOLCANIC VENTS.

Some of these products of volcanic action are of considerable commercial value. At Vulcano regular chemical works have been established in the crater of the volcano, by an enterprising Scotch firm, a great number of workmen being engaged in collecting the materials which are deposited around the fissures, and are renewed by the volcanic action almost as soon as they are removed. In [fig. 6], I have given a sketch of this singular spot, taken from the high ground of the neighbouring Island of Lipari. From the village at the foot of the volcano, where the workmen live, a zig-zag road has been constructed leading up the side, and down into the crater of the volcano. On this road, workmen and mules, laden with the various volcanic materials, may be seen constantly passing up and down.

Fig. 6.—View of Vulcano, with Vulcanello in the foreground taken from the south end of the Island of Lipari.

Vulcano appears to have been frequently in a state of violent eruption during the past 2,000 years—the last great outburst having taken place in 1786. In 1873 the activity in the crater of Vulcano suddenly became more pronounced in character, and the workmen hastened to escape from the dangerous spot, but, before they could do so, several of them were severely injured by the explosions. After this outburst, which did not prove to be of very violent character, the quantity of gases issuing from the fissures in the crater was for a time much greater than before, and the productiveness of these great natural chemical works was proportionately increased: but eventually the action died out almost entirely. The chief products of Vulcano which are of commercial value, are sal-ammoniac, sulphur, and boracic acid. At one time it was even contemplated that great leaden chambers should be erected over the principal fissures at the bottom of the crater of Vulcano, in which chambers the volatile materials might be condensed and collected. The change in the condition of the volcano has unfortunately prevented the carrying out of this bold project.

Besides the volatile substances which issue from volcanic vents, mingling with the atmosphere or condensing upon their sides, there are also many solid materials ejected, and these may accumulate around the orifices, till they build up mountains of vast dimensions, like Etna, Teneriffe, and Chimborazo. Some of these solid materials are evidently fragments of the rock-masses, through which the volcanic fissure has been rent; these fragments have been carried upwards by the force of the steam-blast and scattered over the sides of the volcano. But the principal portion of the solid materials ejected from volcanic orifices consists of matter which has been extruded from sources far beneath the surface, in a highly-heated and fluid or semi-fluid condition.

EJECTED ROCK-FRAGMENTS.

The fragments torn from the sides of volcanic fissures consist of the rocks through which the eruptive forces may happen to have opened their way; pieces of sandstone, limestone, slate, granite, &c., are thus frequently found in considerable numbers among materials which build up volcanic mountains. Thus, some of the volcanic cones in the Eifel are very largely made up of fragments of slate, which have been torn from the sides of the vents by the uprushing currents of steam. At Vesuvius masses of limestone are frequently ejected, and may be picked up all over the slopes of the mountains. These limestone-fragments frequently contain fossils, and Professor Guiscardi, of Naples, has been able to collect several hundred species of shells, transported thus by volcanic action from the rock-masses which form the foundation of the volcano of Vesuvius. The action of water at a high temperature, and under such enormous pressure as must exist beneath volcanic mountains, has often produced changes in the rocks of which fragments are ejected from volcanic vents. The so-called 'lava' ornaments, which are so extensively sold at Naples, are not made from the materials to which geologists apply that name, but from the fragments of altered limestone that have been torn from the rocks beneath the mountain, and scattered by the eruptive forces all over its sides. The chemical action of the superheated and highly-compressed steam on the rocks beneath volcanoes frequently results in the formation of beautifully crystallised minerals. Such crystallised minerals abound in the rock-fragments scattered over the sides of Vesuvius and other volcanoes, both active and extinct. They have been formed in the great chemical laboratories which exist beneath the volcano, and have been brought to the surface by the action of the steam-jets issuing from its fissures.

Of still greater interest are those materials which issue from volcanic orifices in an incandescent, and often in a molten, condition, and which are evidently derived from sources far below the earth's surface. It is to these materials that the name of 'lavas' is properly applied.

Lavas present a general resemblance to the slags and clinkers which are formed in our furnaces and brick-kilns, and consist, like them, of various stony substances which have been more or less perfectly fused. When we come to study the chemical composition and the microscopical structure of lavas, however, we shall find that there are many respects in which they differ entirely from these artificial products.

Let us first consider the facts which are taught us concerning the nature and origin of lavas, by a chemical analysis of them.

CHEMICAL COMPOSITION OF LAVAS.

Of the sixty-five or seventy chemical elements, only a very small number occur at all commonly in lavas. Eight elements, indeed, make up the great mass of all lavas—these are oxygen, silicon, aluminium, magnesium, calcium, iron, sodium, and potassium. But even these eight elements are present in very unequal proportions. Oxygen makes up nearly one-half the weight of all lavas. Almost all the other elements found in lavas exist in combination with oxygen, so that lavas consist entirely of what chemists call 'oxides.' This is a most remarkable circumstance, which, as we shall presently see, is of great significance. The metalloid silicon makes up about one-fourth of the weight of most lavas, and the metal aluminium about one-tenth. The other five elements vary greatly in their relative proportions in different lavas.

In all lavas the substance which forms the greatest part of the mass is the compound of oxygen and silicon, known as silica or silicic acid. In its pure form, this substance is familiar to us as quartz, or rock-crystal and flint. Silica is present in all lavas in proportions which vary from one-half to four-fifths of the whole mass. Now, this substance, silica, has the property of forming more complex compounds by uniting with the other oxides present in lavas—namely, the oxides of aluminium, magnesium, calcium, iron, potassium, and sodium. Silica is called by chemists an acid, the other oxides in lavas are termed bases, and the compounds of silica with the bases are known as silicates. Hence we see that lavas are composed of a number of different silicates—the silicates of aluminium, magnesium, calcium, iron, potassium, and sodium.

The above statements will perhaps be made clearer by the accompanying table from which it will be seen that lavas are compounds in varying proportions of six kinds of salts—namely, the silicates of alumina, magnesia, lime, iron, potash, and soda.

Composition of Lavas.

Elements		Binary Compounds		Salts
Oxygen		Acid	Bases
	Silicon	Silica—	┐
	Aluminum		┠—Alumina	" " Alumina
	Magnesium		┠—Magnesia	" " Magnesia
	Calcium		┠—Lime	" " Lime
	Iron		┠—Iron	" " Iron
	Potassium		┠—Potash	" " Potash
	Sodium		┠—Soda	" " Soda

Now, in some lavas the acid constituent, or silica, is present in much larger proportions than in others. Those lavas with a large proportion of silica are called 'acid lavas,' those with a lower percentage of silica, and therefore a higher proportion of the bases, are known as the 'basic lavas.' It is convenient to employ the term 'intermediate lavas' for those in which the proportion of silica is lower than in the acid lavas, and the proportion of the bases is lower than in the basic lavas.

The acid lavas contain from 66 to 80 per cent, of silica; they are poor in lime, magnesia, and oxide of iron, but rich in potash and soda. The basic lavas contain from 45 to 55 per cent, of silica; they are rich in magnesia, lime, and oxide of iron, but poor in soda and potash. In the intermediate lavas the proportion of silica varies from 55 to 66 per cent.

As the basic-lavas contain a larger proportion of oxide of iron and other heavy oxides than the acid-lavas, the former have usually a higher specific gravity than the latter; it is, indeed, possible in most cases to distinguish between these different varieties by simply weighing them in water and in air.

DIFFERENT KINDS OF LAVA.

The basic lavas are usually of much darker colour than the add lavas—the terms acid lavas, intermediate lavas, and basic lavas correspond indeed pretty closely with the names trachytes, greystones and basalt, which were given to the varieties of lavas by the older writers on volcanoes, at a time when their chemical constitution had not been accurately studied. Fresh lavas of acid composition are usually nearly white in colour, intermediate lavas are of various tints of grey, and basic lavas nearly black. It must be remembered, however, that colour is one of the least persistent, and therefore one of the least valuable, characters by means of which rocks can be discriminated, and also that by exposure to the influence of the atmospheric moisture the iron present in all lavas is affected, and the lavas belonging to all classes, when weathered, assume reddish and reddish-brown tints.

Geologists have devised a great number of names for the various kinds of lava which have been found occurring round volcanic vents in different parts of the world, and the study of these varieties is full of interest. For our present purpose, however, it will be sufficient to state that they nearly all fall into five great groups, known as the Rhyolites, the Trachytes, the Andesites, the Phonolites, and the Basalts. The Rhyolites are acid lavas, the Basalts are basic lavas, and the Trachytes, Andesites, and Phonolites, different kinds of intermediate lavas, distinguished by the particular minerals which they contain.

Before we part from this subject of the classification of lavas according to their chemical composition, it will be well to point out that there exists a small group of lavas which stand quite by themselves, and cannot be referred to either of the classes we have indicated. They contain a smaller proportion of silica, and a much larger proportion of magnesia and oxide of iron than the other lavas, and may be made to constitute a small sub-group, to which we may apply the term of 'ultra-basic lavas.' Although much less widely distributed than the other varieties, they are, in some respects, as we shall presently have to point out, of far greater interest to the geologist than all the other kinds of lavas.

MINUTE STRUCTURE OF LAVAS.

We will now proceed to consider the facts which are brought to light concerning the nature of lavas, when they are studied by the aid of the microscope. Although most lavas appear at first sight to be opaque substances, yet it is easy to prepare slices of them which are sufficiently thin to transmit light. In such thin transparent slices we are able to make out, by the aid of the microscope, certain very interesting details of structure, which afford new and important evidence bearing on the mode of origin of these rocks.

Host lavas are capable of being melted by the heat of our furnaces; but the different kinds of lava vary greatly in the degree of their fusibility. The basic lavas, or those with the smallest proportion of silica, are usually much more easily fusible than those which contain a high percentage of silica, the add lavas.

Now, it is a very noteworthy circumstance, that when a lava is artificially fused it assumes on cooling very different physical characters to those which were presented by the original rock.

If we examine the freshly-broken surface of a piece of lava, we shall, in most cases, find that it contains a great number of those regular-shaped bodies which we call crystals; in some cases these crystals are so small as to be scarcely visible to the naked eye, in others they may be an inch or more in length. Most lavas are thus seen to be largely made up of crystals of different minerals. The minerals which are usually contained in lavas are quartz, the various kinds of felspar, augite, hornblende, the different kinds of mica, olivine, and magnetite.

But when a piece of lava is melted in a furnace, all these crystalline minerals disappear, and the resulting product is the homogeneous substance which we call glass. If, as many suppose, lavas acquire the fluidity which they possess when issuing from volcanic vents as the result of simple fusion it is strange that artificially fused lavas do not agree more closely in character with the natural products.

A careful examination of different kinds of lavas, however, will show that they vary very greatly in character among themselves. Some lavas are as perfectly glassy in structure as those which have been artificially fused, while others contain great numbers of crystals, which may sometimes be of very large size.

If we prepare thin transparent slices of these different kinds of lavas, and examine them by the aid of the microscope, we shall find that lavas are made up of two kinds of materials, a base or groundmass of a glassy character, and distinct crystals of different minerals, which are irregularly distributed through this glassy base, like the raisins, currants, and pieces of candied peel in a cake. In some cases the glassy base makes up the whole mass of the rock; in others, smaller or larger numbers of crystals are seen to be scattered through a glassy base; while in others again the crystals are so numerous that the presence of an intervening glassy base or groundmass can only be detected by the aid of the microscope.

STUDY OF LAVAS WITH THE MICROSCOPE.

If thin slices of the glassy materials of lavas be examined with high magnifying powers, new and interesting facts are revealed. Through the midst of the clear glassy substance cloudy patches are seen to be diffused; and, if we examine them with a still higher power, these cloudy patches resolve themselves into innumerable particles, some transparent and others opaque, having very definite outlines. At the same time fresh cloudy patches are brought into view, which can only be resolved by yet higher powers of the microscope. In examining these natural glasses by the aid of the microscope, we are forcibly reminded of what occurs when the 'Milky Way' and some other parts of the heavens are studied with a telescope. As the power of the instrument is increased the nebulous patches are resolved into distinct stars, but fresh nebulous masses come into view, which are in turn resolved into stars, when higher powers of the instrument are employed.

In the Frontispiece, No. 1 illustrates the appearance presented by these volcanic glasses when examined with a high power of a microscope. Through a glassy base is seen a number of diffused nebulous patches, which are in places resolved into definite particles.

These minute particles of definite form, which the microscope has revealed in the midst of the glassy portions of lava, have received the name of microliths, or crystallites. The study of the characters and mode of arrangement of these microliths or crystallites has in recent years thrown much new light on the interesting problems presented by lavas.

In some glassy lavas the microliths or crystallites, instead of being indiscriminately diffused through the mass of the base or groundmass, are found to be collected together into groups of very definite form. In No. 2 of the Frontispiece we have a section of a glassy rock in which the crystallites have united together, so as to build up groups presenting the most striking resemblance to fronds of ferns. Around these groups spaces of dear glass have been left by the gathering up of the crystallites, which in other parts of the mass are seen to be equally diffused through it. In this formation of groups of microliths we cannot but recognise the action of those crystalline forces, which on frosty mornings cover our windows with a mimic vegetation composed of icy particles.

In other cases, again, the crystallites scattered through the glassy portions of lavas unite in radial groups about certain centres, and thus build up globular masses to which the name of 'sphærulites' has been given. No. 3 in the Frontispiece illustrates the formation of these sphærulites.

Now, a careful study of the microliths or crystallites has proved that they are the minute elements of which those wonderfully beautiful objects which we call crystals are built up. In some cases we can see that the crystallites are becoming united together in positions determined by mathematical laws, and the group is gradually assuming the outward form and internal structure of a crystal. In other cases crystals may be found which are undergoing a disintegrating action, and are then seen to be made up of minute elements similar to the crystallites or microliths of glassy rocks.

CRYSTALLITES AND CRYSTALS.

The conclusion is confirmed by the fact that if we take an artificially fused lava and allow it to cool slowly, it will be found that the glassy mass into which it has resolved itself contains numerous crystallites. If the cooling process be still further prolonged, these crystallites will be found to have united themselves into definite groups, and sometimes distinct crystals are formed in the mass; under these circumstances the rock frequently loses its glassy appearance and assumes a stony character.

In connection with this subject, it may be mentioned that some years ago a very ingenious invention was submitted to trial in the Works of the Messrs. Chance, of Birmingham. It had been suggested that if certain lavas of easy fusibility were melted and poured into moulds, we might thus obtain elaborately ornamented stone-work, composed of the hardest material, without the labour of the mason. The molten rock when quickly cooled was found to assume the form of a black glass, but when very slowly cooled passed into a stony material. Unfortunately, it was found that this material did not withstand the weather like ordinary building stones, and, in consequence, the manufacture had to be abandoned.

Now, the study of the products of volcanoes has led geologists to recognise the true relations between glassy and crystalline rocks.

In the amorphous mixture of various silicates which compose a glass, chemical affinity causes the separation of certain portions of definite composition, and these form the microliths or elements of which different crystalline minerals are built up. Under the influence of the crystalline forces, there is a great shaking or agitation in the mass, and the microliths of similar kind come together and become united, like the fragments in Ezekiel's valley of dry bones.

Although we cannot see this process taking place under our eyes, in a mass of lava, yet we may study specimens in which the action has been arrested in its different stages. In order to understand the development of an acorn into an oak-tree, it is not necessary to watch the whole series of changes in a particular case. A visit to an oak-thicket, in which illustrations of every stage of the transformation may be found, will afford us equally certain information on the subject.

In the same way by the examination of such a series of rock-sections as that represented in the Frontispiece, we may understand how, in the midst of a mass of mixed silicates constituting a natural glass, the separation of microliths takes place; these unite into groups which are the skeletons of crystals, and finally, by the filling up of the empty spaces in these skeletons, complete crystals are built up. The series of operations may, however, be interrupted at any stage, and this stage we may have the chance of studying.

GLASSY AND CRYSTALLINE LAVAS.

We are able, as we shall show in a future chapter, to examine many rock-masses that have evidently formed the reservoirs from which volcanoes have been supplied, and others that fill up the ducts which constituted the means of communication between these subterranean reservoirs, and the surface of the earth. Now in these subterranean regions the lavas have been placed under conditions especially favourable for the action of the crystalline forces—they must have cooled with extreme slowness, and they must have been under an enormous pressure, produced in part by the weight of the superincumbent rocks, and in part by the expansive force of the imprisoned steam. We are not, therefore, surprised to find that in these subterranean regions, the lavas, while retaining the same chemical composition, have assumed a much more perfectly crystalline condition. In some cases, indeed, the whole rock has become a mass of crystals without any base or groundmass at all.

An examination of the Frontispiece will illustrate this perfect gradation from the glassy to the crystalline condition of lavas. No. 1 represents a glass through which microliths or crystallites of different dimensions and character are diffused. In Nos. 2 and 3, these crystallites have united to form regular groups. In No. 4, which may be taken as typical of the features presented by most lavas, we have a glassy groundmass containing microliths (a 'crypto-crystalline base'), through which distinct crystals are distributed. Nos. 5 and 6 illustrate the characters presented by lavas which have consolidated at considerable depths beneath the surface; in the former we have a mans of small crystals (a 'micro-crystalline base') with larger crystals scattered through it; while the latter is entirely made up of large crystals without any trace of a base or groundmass.

Now, as all lavas are found sometimes assuming the glassy condition at the surface, so when seen in the masses which have consolidated with extreme slowness, and under great pressure, in subterranean regions, the same materials are found in the condition of a rock which is built up entirely of crystals. Chemists have found that artificial mixtures of silicates in which soda and potash are present in considerable quantities, have a great tendency to assume the glassy condition on cooling from a state of fusion, and glass manufacturers are always careful to use considerable proportions of the alkalis as ingredients, in making glass. It is found, in like manner, that those lavas which contain the largest portion of the silicates of soda and potash (the 'acid lavas') most frequently assume the condition of a natural glass.

Geologists have given distinct names to the glassy and the perfectly crystalline conditions of the different kinds of lavas, the glassy varieties being found in masses which have cooled rapidly near the surface, and the crystalline varieties in masses which have cooled slowly at great depths. The names of these two conditions of the five great classes into which we have divided lavas are as follows:—

HIGHLY CRYSTALLINE IGNEOUS ROCKS.

Crystalline Forms.	Lavas.		Glassy Forms.
Granite	Rhyolite		Obsidian.
Syenite	Trachyte
Diorite	Andesite
Miascite	Phonolite
Gabbro	Basalt		Tachylyte.

As vitreous rocks have little in their general appearance to distinguish them from one another, the glassy forms of the first four classes of lava have not hitherto received distinct names, but have been confounded together under the name of obsidian. If we determine the specific gravities of rocks having the same composition but different structures, we shall find that they become heavier in proportion as the crystalline structure is developed in them. Thus gabbro is heavier, but tachylyte is lighter than basalt, bulk for bulk, though all have the same chemical composition.

Nor are the crystals contained in lavas less worthy of careful study, by the aid of the microscope, than the more or less glassy groundmass in which they are embedded. Mr. Sorby has shown that the crystals found in lavas, exhibit many interesting points of difference from those which separate out in the midst of a mass of the same rock, when it has been artificially melted and slowly cooled. There are other facts which also point to the conclusion that, while the glassy groundmass of lavas may have been formed by cooling from a state of fusion, the larger and well-formed crystals in these lavas must have been formed under other and very different conditions.

The larger crystals in lavas exhibit evidence of having been slowly built up in the midst of a glassy mass, containing crystallites and small crystals. We can frequently detect evidence of the interruptions which have occurred in the growth of these crystals in the concentric zones of different colour or texture which they exhibit; and portions of the glassy base or groundmass are often found to have been caught up and enclosed in these crystals during their growth.

But when we find, as in the porphyritic pitchstones, a glassy base containing only minute crystallites, through which large and perfectly formed crystals are distributed, we can scarcely doubt that the minute crystallites and the larger crystals have separated from the base under very different conditions. This is indicated by the bet that we detect in these cases no connecting links between the embryo microliths and the perfect crystals; and a confirmation of the conclusion is seen in the circumstance that many of the crystals are found to have suffered injury as if from transport, their edges and angles being rounded and abraded, and portions being occasionally broken off from them.

Hence we are led to conclude that the larger crystals in lavas were probably separated from the amorphous mass in the subterranean reservoirs beneath the volcano, and were carried up to the surface in the midst of the liquefied glassy material which forms the groundmass of lavas. When we come to examine these crystals more closely, we find that certain very curious phenomena are exhibited by them which lend powerful support to this conclusion.

Fig. 7.—Minute Cavities, containing Liquids, in the Crystals of Rocks.

LIQUID CAVITIES IN CRYSTALS.

It is found convenient by geologists to designate those rocks which have consolidated in deep-seated portions of the earth's crust as Platonic Rocks, confining the name of Volcanic rocks to those consolidating At the surface; but Plutonic and Volcanic Rocks shade into one another by the most insensible gradations.

When the crystals embedded in granitic rocks, and in some lavas, are examined with the higher powers of the microscope, they are frequently seen to contain great numbers of excessively minute cavities. Each of these cavities resembles a small spirit-level, having a quantity of liquid and a bubble of gas within it. In [fig. 7] we have given a series of drawings of these cavities in crystals as seen under a high power of the microscope. In No. 1 a group of such cavities is represented, one of which is full of liquid, while two others are quite empty; the remaining cavities all contain a liquid with a moving bubble of gas. In No. 2 two larger cavities are shown, containing a liquid and a bubble of gas; and it will be seen from these how varied in form these cavities sometimes are. In Nos. 3, 4 and 6 the liquid in the cavities contains, besides the bubbles, several, minute crystals; and in No. 6 we have a cavity containing two liquids and a bubble.

In the largest of such cavities the bubble is seen to change its place so as always to lie at the upper side of the cavity, when the position of the latter is altered, just as in a spirit-level. But in the smallest cavities the bubbles appear to be endowed with a power of spontaneous movement; like imprisoned creatures trying to escape, these bubbles are seen continually oscillating from side to side and from end to end of the cavities which enclose them. In [fig. 8] a minute cavity containing a liquid and bubble is shown, the path pursued by the latter in its wonderful gyrations being indicated by the dark line. These cavities are exceedingly minute, and so numerous that in some crystals there must be millions of them present; indeed, in certain cases, as we increase the magnifying power of our microscopes, new and smaller cavities continually become visible. It has been estimated that in some instances the number of these minute liquid-cavities in the crystals of rocks amounts to from one thousand millions to ten thousand millions in a cubic inch of space.

Fig. 8—Minute Liquid-cavity in a Crystal, with a moving Bubble. (The path of the bubble is indicated by the dark line.)

NATURE OF LIQUIDS IN CAVITIES.

What is the nature of the liquids which are thus imprisoned in these cavities contained in the crystals of lavas and granites? Careful experiments have given a conclusive answer to this question. In many cases the liquid is water, usually containing considerable quantities of saline matter dissolved in it. Sometimes the saline matters are present in such abundance that they cannot all pass into solution, but crystallise out, as in [fig. 7]—Nos. 3, 4, 5—where cubic crystals of the chlorides of sodium and potassium are seen floating in the liquid; in other cases the liquid is a hydrocarbon like the mineral oil which is present in great abundance in deep-seated rocks in many parts of the globe. But in some other cases the liquid contained in the cavities of crystals is found to be one which could scarcely be anticipated to occur under such circumstances—the gas known as carbonic add, which under extreme pressure can be reduced to a liquid condition. In cavities containing liquefied carbonic acid, if the rock be warmed up to 86° or 90° Fahrenheit the bubble suddenly vanishes, sometimes with an appearance like ebullition or boiling, as represented in [fig. 9]. Now the temperature which we have indicated is the 'critical point' of carbonic acid, and above that temperature it cannot exist in a liquid condition, however great may be the pressure to which it is subjected. The liquid has been converted into a gas which completely fills the cavity. The carbonic acid in the cavities of crystals has frequently been isolated and its nature placed beyond doubt by spectroscopic and ordinary chemical tests.

The presence of these liquids in the cavities of crystals clearly proves that the latter must have been formed under enormous pressure—a pressure sufficiently great to reduce, not only steam, but also volatile hydrocarbons and even gaseous carbonic acid, to the bulk of a liquid.

Fig. 9.—Cavity in Crystal containing Carbonic-Acid Gas at a temperature of 86° F., and passing from the liquid to the gaseous condition.

Such conditions of enormous pressure we may infer to exist in the deep-seated reservoirs beneath volcanoes, where, besides the weight of the superincumbent rock-masses, we have the compressing force of great quantities of elastic vapour held in confinement. The crystals of which granitic rocks are entirely built up exhibit clear evidence of having been all formed under these conditions of enormous pressure. The glassy base or groundmass of lavas, on the other hand, presents all the characters of materials that have cooled from a state of fusion. Most lavas consist in part of crystals, exhibiting fluid-cavities like those present in granite, and in part of a base, which has evidently been formed by the cooling of a fused mass. We are therefore justified in concluding that the crystals have been formed in subterranean recesses, and that the groundmass or base has consolidated at the surface. The bearing of these conclusions upon some of the great problems presented by volcanoes we shall have occasion to point out in the sequel.

CAUSE OF MOVEMENT OF BUBBLES.

One of the most interesting inquiries suggested by the study of the liquid-cavities in volcanic rocks is that of the cause of the apparently spontaneous movement of the bubbles which we have described as taking place in some of the smaller of them. The ingenious experiments of Mr. Noel Hartley have suggested to Professor Stokes an explanation which is probably the true one. It appears that these minute globes of vapour are in such a state of unstable equilibrium as to be affected by the smallest changes of temperature, and that the variations in the heat of the atmosphere, due to currents of air and the movement of warm or cold bodies through it, are sufficient to cause the oscillation of these sensitively poised bubbles.

The short account which we have been able to give in the foregoing pages of the researches that have been carried on concerning the nature of the materials ejected from volcanoes will serve to show that these investigations have already made known many facts of great interest, and that the farther pursuit of them is full of the highest promise. To the scientific worker no subject is too vast for his research, no object so minute as to be unworthy of his most patient study. In some of our future inquiries concerning the nature of volcanic action, we shall be led to an investigation of the phenomena displayed in the sun, moon, comets and other great bodies of the universe; but another road to truths of the same grandeur and importance is found, as we have seen, in an examination of the mode of development of crystallites, and a study of the materials contained in the microscopic cavities of the minutest crystals.

CHAPTER IV.
THE DISTRIBUTION OF THE MATERIALS EJECTED FROM VOLCANIC VENTS.

The escape of great quantities of steam and other gases from the midst of a mass of fluid or semi-fluid lava gives rise to the formation of vast quantities of froth or foam upon its surface. This froth or foam, which is formed upon the surface of lava by the escape of gaseous matters from within it, is made up of portions of the lava distended into vesicles, in the same way that bubbles are formed on the surface of water. It bears precisely the same relation to the liquid mass of lava that the white crest of foam upon an advancing wave does to the sea-water, from the bubbles of which it is formed.

This froth upon the surface of lavas varies greatly in character according to the nature of the material from which it is formed. In the majority of cases the lavas consist, as we have seen, of a mass of crystals floating in a liquid magma, and the distension of such a mass by the escape of steam from its midst gives rise to the formation of the rough cindery-looking material to which the name of 'scoria' is applied. But when the lava contains no ready-formed crystals, but consists entirely of a glassy substance in a more or less perfect state of fusion, the liberation of steam gives rise to the formation of the beautiful material known as 'pumice.' Pumice consists of a mass of minute glass bubbles; these bubbles have not usually, however, retained their globular form, but have been elongated in one direction through the movement of the mass while it was still in a plastic state.

The steam frequently escapes from lava with such violence that the froth or scum on its surface is broken up and scattered in all directions, as the foam crests of waves are dispersed by the wind during a storm. In this way fragments of scoria or pumice are often thrown to the height of many hundreds or thousands of feet into the atmosphere, as we have seen is the case at Stromboli and Vesuvius. Indeed, during violent eruptions, a continuous upward discharge of these fragments is maintained, the ragged cindery masses hurtling one another in the atmosphere, as they are shot perpendicularly upwards to an enormous height and fall back into the vent; or they may rise obliquely and describe curves so as to descend outside the orifice from which they were ejected.

FINENESS OF VOLCANIC DUST.

During their upward discharge and downward fall, the cindery fragments are by attrition continually reduced to smaller dimensions. The noise made by these fragments, as they strike against one another in the air during their rise and fall, is one of the most noteworthy accompaniments of volcanic eruptions. It has been noticed that in many cases there is a constant diminution in the size of the fragments ejected during a volcanic outburst, this being doubtless due to the friction of the masses as they are ejected and re-ejected from the vent. Thus it is related by Mr. Poulett Scrope, who watched the Vesuvian eruption of 1822, which lasted for nearly a month, that during the earlier stages of the outburst fragments of enormous size were thrown out of the crater, but by constant re-ejection these were gradually reduced in size, till at last only the most impalpable dust issued from the vent. This dust filled the atmosphere, producing in the city of Naples 'a darkness that might be felt,' and so excessively finely divided was it, that it penetrated into all drawers, boxes, and the most closely fastened receptacles, filling them completely. Mr. Whymper relates that, while standing on the summit of Chimborazo, he witnessed an eruption of Cotopaxi, which is distant more than fifty miles from the former mountain. The fine volcanic dust fell in great quantities around him, and he estimated that no less than two millions of tons must have been ejected during this slight outburst. Professor Bonney has examined this volcanic dust from Cotopaxi, and calculates that it would take from 4,000 to 25,000 particles to make up a grain in weight.

Various names have been given by geologists to the fragments ejected from volcanic vents, which, as we have seen, differ greatly in their dimensions and other characters. Sometimes masses of more or less fluid lava are flung bodily to a great height in the atmosphere. During their rise and fall these masses are caused to rotate, and in consequence assume a globular or spheroidal form. The water imprisoned in these masses, during their passage through the atmosphere, tends to expand into steam, and they become more or less completely distended with bubbles. Such masses, which sometimes assume very regular and striking forms, are known as 'volcanic bombs.' Many volcanic bombs have a solid nucleus of refractory materials. The large, rough, angular, cindery-looking fragments are termed 'scoriæ.' When reduced to the dimensions of a marble or pea they are usually called by the Italian name of 'lapilli.' The still finer materials are known as volcanic sand and dust.

There are, however, two names which are frequently applied to these fragmentary materials ejected from volcanoes, which are perhaps liable to give rise to misconception. These are the terms 'cinders' and 'ashes.' It must be remembered that the scoriæ or cindery-looking masses are not, like the cinders of our fires, the product of the partial combustion of a material containing inflammable gases, but are, like the clinkers of furnaces and brick-kilns, portions of partially vitrified and fused rock distended by gases. So, too, volcanic ashes only resemble the ashes of our grates in being very finely divided; they are not, like the latter, the incombustible residue of a mass which has been burnt.

VOLCANIC BOMBS AND PELE'S HAIR.

The glassy lavas, when distended by escaping gases, give rise to the formation of pumice, the white colour of which, as in the case of the foam of a wave, is due to the reflection of a portion of the light in its frequent passage from one medium to another—in this case from air to glass, and from glass to air. The volcanic bombs formed from glassy lavas are often of especially beautiful and regular forms. Sometimes the passage of steam through a mass of molten glass produces large quantities of a material resembling spun glass. Small particles or shots of the glass are carried into the air and leave behind them thin, glassy filaments like a tail. At the volcano of Kilauea in Hawaii this filamentous volcanic glass is abundantly produced, and is known as 'Pele's Hair'—Pele being the name of the goddess of the mountain. Birds' nests are sometimes found composed of this beautiful material. In recent years an artificial substance similar to this Pele's hair has been extensively manufactured by passing jets of steam through the molten slag of iron-furnaces; it resembles cotton-wool, but is made up of fine threads of glass, and is employed for the packing of boilers and other purposes.

The very finely-divided volcanic dust is often borne to enormous distances from the volcano out of which it has been ejected. The force of the steam-current carrying the fragments into the atmosphere is often so great that they rise to the height of several miles above the mountain. Here they may actually pass into the upper currents of the atmosphere and be borne away to the distance of many hundreds or thousands of miles. Hence it is not an unusual circumstance for vessels at sea to encounter at great distances from land falling showers of this finely divided, volcanic dust. We sometimes meet with this far-travelled, volcanic dust under very unexpected circumstances. Thus, in the spring of 1875 I had occasion to visit Prof. Vom Rath of Bonn, who showed me a quantity of fine volcanic dust which had during the past winter fallen in considerable quantities in certain parts of Norway. This dust, upon microscopic examination, proved to be so similar to what was known to be frequently ejected from the Icelandic volcanoes that a strong presumption was raised that volcanic outbursts had been going on in that island. On returning to England I found that the first steamer of the season had just reached Leith from Iceland, bringing the intelligence that very violent eruptions had taken place during the preceding months.

DISPERSION OF PUMICE AND VOLCANIC DUST.

This finely-divided volcanic dust is thus carried by the winds and spread over every part of the ocean. Everyone is familiar with the fact that pumice floats upon water; this it does, not because it is a material specifically lighter than water, but because cavities filled with air make up a great part of its bulk. If we pulverise pumice, we find the powder sinks readily in water, but the rock in its natural condition floats for the same reason that an iron ship does—because of the air-chambers which it encloses. When this pumice is ejected from a volcano and falls into a river or the ocean, it floats for a long time, till decomposition causes the breaking down of the thin glassy partitions between the air chambers, and causes the admission of water into the latter, by which means the whole mass gets water-logged. Near the Liparis and other volcanic islands the sea is sometimes covered with fragments of pumice to such an extent that it is difficult for a boat to make progress through it, and the same substance is frequently found floating in the open ocean and is cast up on every shore.

During the year 1878 masses of floating pumice were reported as existing in the vicinity of the Solomon Isles, and covering the surface of the sea to such extent that it took ships three days to force their way through them. Sometimes these masses of pumice accumulate in such quantities along coasts that it is difficult to determine the position of the shore within a mile or two, as we may land and walk about on the great floating raft of pumice. Now, recent deep-sea soundings, carried on in the 'Challenger' and other vessels, have shown that the bottom of the deepest portion of the ocean, far away from the land, is covered with these volcanic materials which have been carried through the air or floated on the surface of the ocean. To these deeper parts of the ocean no sediments carried down by the rivers are borne, and the remains of calcareous organisms are, in these abysses, soon dissolved; under such conditions, therefore, almost the only material accumulating on the sea bottom is the ubiquitous wind- and wave-borne volcanic products. These particles of volcanic dust and fragments of pumice by their disintegration give rise to a clayey material, and the oxidation of the magnetite, which all lavas contain, communicates to the mass a reddish tint. This appears to be the true origin of those masses of 'red-clay' which, according to recent researches, are found to cover all the deeper parts of the ocean, but which probably attain to no great thickness.

But while some portion of the materials ejected from volcanoes may thus be carried by winds and waves, so as to be dispersed over every part of the land and the ocean-bed, another, and in most cases by far the largest, portion of these ejections falls around the volcanic vent itself. It is by the constant accumulation of these ejected materials that such great mountain masses as Etna, Teneriffe, Fusiyama, and Chimborazo have been gradually built up around centres of volcanic action.

There are cases in which the formation of volcanic mountains on a small scale has actually been observed by trustworthy witnesses. There are other cases in which volcanic mountains of larger size can be shown to have increased in height and bulk by the fall upon their sides and summits of fragmentary materials ejected from the volcanic vent. In all cases the examination of these mountain-masses leads to the conclusion that they are entirely built up of just such materials as we constantly see thrown out of volcanoes during eruption.

FORMATION OF VOLCANIC MOUNTAINS.

Thus we are led to the conclusion that all volcanic mountains are nothing but heaps of materials ejected from fissures in the earth's crust, the smaller ones having been formed during a single volcanic outburst, the larger ones being the result of repeated eruptions from the same orifice which may, in some cases, have continued in action for tens or hundreds of thousands of years.

No observer has done such useful work in connection with the study of the mode of formation of volcanic mountains as our countryman, Sir William Hamilton, who was ambassador at Naples from 1764 to 1800, and made the best possible use of his opportunities for examining the numerous volcanoes in Southern Italy.

A little to the west of the town of Puzzuoli on the Bay of Naples there stands a conical hill rising to the height of 440 feet above the level of the Mediterranean, and covering an area more than half a mile in diameter. Now we have the most conclusive evidence that in ancient times no such hill existed on this site, which was partly occupied by the Lucrine Lake, and the fact is recognised in the name which the hill bears, that of Monte Nuovo, or the 'New Mountain.' See [fig. 10].

Sir William Hamilton rendered admirable service to science by collecting all the contemporary records relating to this interesting case, and he was able to prove, by the testimony of several intelligent and trustworthy witnesses, that during the week following the 29th of September, 1538, this hill had gradually been formed of materials ejected from a volcanic vent which had opened upon this site.

Fig. 10. Monte Nuovo (440 ft. high) on the shores of the Bay of Naples.

HISTORY OF THE FORMATION OF MONTE NUOVO.

The records collected by Hamilton with others which have been discovered since his death prove most conclusively the following facts. During more than two years, the country round was affected by earthquakes, which gradually increased in intensity and attained their climax in the month of September 1538; on the 27th and 28th of that month these earthquake shocks are said to have been felt almost continuously day and night. About 8 o'clock on the morning of the 29th, a depression of the ground was noticed on the site of the future hill, and from this depression, water, which was at first cold and afterwards tepid, began to issue. Four hours afterwards the ground was seen to swell up and open, forming a gaping fissure, within which incandescent matter was visible. From this fissure numerous masses of stone, some of them 'as large as an ox,' with vast quantities of pumice and mud, were thrown: up to a great height, and these falling upon the sides of the vent formed a great mound. This violent ejection of materials continued for two days and nights, and on the third day a very considerable hill was seen to have been built up by the falling fragments, and this hill was climbed by some of the eye-witnesses of the eruption. The next day the ejections were resumed, and many persons who had ventured on the hill were injured, and several killed by the falling stones. The later ejections were however of less violence than the earlier ones, and seem to have died out on the seventh or eighth day after the beginning of the outburst. The great mass of this considerable hill would appear, according to the accounts which have been preserved, to have been built up by the materials which were ejected during two days and nights.

Monte Nuovo is a hill of truncated conical form, which rises to the height of 440 feet above the waters of the Mediterranean, and is now covered with thickets of stone-pine. The hill is entirely made up of volcanic scoriæ, lapilli, and dust, and the sloping sides have evidently been produced by these fragmentary materials sliding over one another till they attained the angle of rest; just as happens with the earth and stones tipped from railway-waggons during the construction of an embankment. In the centre of this conical hill is a vast circular depression, with steeply sloping sides, which is of such depth that its bottom is but little above the sea-level. This cup-shaped depression is the 'crater' of the volcano, and it has evidently been formed by the explosive action which has thrown out the materials immediately above the vent, and caused them to be accumulated around it.

Fig. 11.—Map of the district around Naples, showing Monte Nuovo and the surrounding volcanoes of older date.

The district lying to the west of Naples, in which the Monte Nuovo is situated, contains a great number of hills, all of which present a most striking similarity to that volcano. All these hills are truncated cones, with larger or smaller circular depressions at their summits, and they axe entirely composed of volcanic scoriæ, lapilli, and dust. Some of these hills are of considerably larger dimensions than the Monte Nuovo, while others are of smaller size, as shown in the annexed map, [fig. 11]. No stranger visiting the district, without previous information upon the subject, would ever suspect the fact that, while all the other hills of the district have existed from time immemorial, and are constantly mentioned in the works of Greek and Roman writers, this particular hill of Monte Nuovo came into existence less than 350 years ago.

OLDER VOLCANOES OF THE CAMPI PHLEGRÆI.

The evidently fused condition of the materials of which these hills are built up is a dear sign of the volcanic action which has taken place in it; and this feet was so fully recognised by the ancients that they called the district the Campi Phlegræi, or 'the Burning Fields,' and regarded one of the circular depressions in it as the entrance to Hades.

It is impossible for anyone to examine this district without being convinced that all the numerous cones and craters which cover it have been formed by the same agency as that by which Monte Nuovo was produced. We have shown that there is the most satisfactory historical evidence as to what that agency was.

Now volcanic cones with craters in their centres occur in great numbers in many parts of the earth's surface. In some districts, like the Auvergne, the Catacecaumene in Asia Minor, and certain parts of New Zealand, these volcanic cones occur by hundreds and thousands. In some instances, these volcanic cones have been formed in historic times, but in the great majority of cases we can only infer their mode of origin from their similarity to others of which the formation has been witnessed.

Most of the smaller volcanic hills, with their craters, have been thrown up during a single eruption from a volcanic fissure; but, as Hamilton conclusively proved, the grandest volcanic mountains must have been produced by frequent repetitions of similar operations upon the same site. For not only are these great volcanic piles found to be entirely composed of materials which have evidently been ejected from volcanic vents, but, when carefully watched, such mountains are found undergoing continual changes in form, by the addition of materials thrown out from the vent, and falling upon their sides.

This fact will be well illustrated by a comparison of the series of drawings of the summit of Vesuvius which were made by Sir William Hamilton in 1767, and which we have copied in [fig. 12]. During the earlier months of that year the summit of the mountain was seen to be of truncated form, a great crater having been originated by the violent outbursts of the preceding year. This condition of the mountain-top is represented in the first figure of the series. The drawing made by Hamilton, on July 8, shows that not only was the outer rim of the great crater being modified in form by the fall of materials upon it, but that in the centre of the crater a small cone was being gradually built up by the quiet ejections which were taking place.

Fig. 12.—Outlines of the Summit of Vesuvius during the Eruption of 1767.
Click on image to view original negative image.

CHANGES IN FORM OF VESUVIUS.

If we compare the drawings made at successive dates, we shall find that the constant showers of falling materials were not only raising the edge of the great crater but were at the same time increasing the size of the small cone inside the crater. By the end of October the small cone had grown to such an extent that its sides were confluent with those of the principal cone, which had thus entirely lost its truncated form and been raised to a much greater height. The comparison of these drawings will be facilitated by the dotted lines, which represent the outline of the top of the mountain at the preceding observation; so that the space between the dotted and the continuous line in each drawing shows the extent to which the bulk of the cone had increased in the interval between two observations.

But, although the general tendency of the action going on at volcanic mountains is to increase their height and bulk by the materials falling upon their summits and aides, it must be remembered that this action does not take place by any means continuously and regularly. Not only are there periods of rest in the activity of the volcano, during which the rain and winds may accomplish a great deal in the way of crumbling down the loose materials of which volcanic mountains are largely built up, but sudden and violent eruptions may in a very short time undo the slow work of years by blowing away the whole summit of the mountain at once. Thus, before the great eruption of 1822, the cone of Vesuvius, by the almost constant ejection of ashes during several years, had been raised to the height of more than 4,000 feet above the level of the sea; but by the terrible outburst which then took place the cone was reduced in height by 400 feet, and a vast crater, which had a diameter of nearly a mile, and a depth of nearly 1,000 feet (see [fig. 13]), was formed at the top of the mountain. The enormous quantity of material thus removed was either distributed over the flanks of the mountain, or, when reduced to a finely comminuted condition, was carried by the wind to the distance of many miles, darkening the air, and coating the surface of the ground with a thick covering of dust.

Fig. 13.—Crater of Vesuvius formed during the eruption in 1822. (It was nearly 1 mile in diameter and 1,000 ft. deep.)

EARLY HISTORY OF VESUVIUS.

The volcano of Vesuvius, although of somewhat insignificant dimensions when compared with the grander volcanic mountains of the globe, possesses great interest for the student of Vulcanology, inasmuch as being situated in the midst of a thickly populated district and in close proximity to the city of Naples, it has attracted much attention during past times, and there is no other volcano concerning which we have so complete a series of historical records. The present cone of Vesuvius, which rises within the great encircling crater-ring of Somma, has a height of about 1,000 feet. But there is undoubted evidence that this cone, to the top of which a railway has recently been constructed for the convenience of tourists, has been entirely built up during the last 1,800 years, and, what is more, that during this period it has been many times almost wholly destroyed and reconstructed.

Nothing is more certain than the bet that the Vesuvius upon which the ancient Romans and the Greek settlers of Southern Italy looked, was a mountain differing entirely in its form and appearance from that with which we are familiar. The Vesuvius known to the ancients was a great truncated cone, having a diameter at its base of eight or nine miles, and a height of about 4,000 feet. The summit of this mountain was formed by a circular depressed plain, nearly three miles in diameter, within which the gladiator Spartacus, with his followers, were besieged by a Roman army. There is no evidence that at this time the volcanic character of the mountain was generally recognised, and its slopes are described by the ancient geographers as being clothed with fertile fields and vineyards, while the hollow at the top was a waste overgrown with wild vines.

Fig. 14.—Crater of Vesuvius in 1756. (From a drawing made on the spot)

But in the year 79 a terrible and unexpected eruption occurred, by which a vast, crateral hollow was formed in the midst of Vesuvius, and all the southern side of the great rim surrounding this crater was broken down. Under the materials ejected during this eruption, the cities of Pompeii, Herculaneum, and Stabiæ were overwhelmed and buried.

Numerous descriptions and drawings enable us to understand how in the midst of the vast crater formed in the year 79 the modern cone has gradually been built up. Fresh eruptions are continually increasing the bulk, or raising the height of the Vesuvian cone.

The accompanying drawings made by Sir William Hamilton enable us to understand the nature of the changes which have been continually taking place at the summit of Vesuvius. The drawing [fig. 14] shows the appearance presented by the crater in the year 1756.

Fig. 15.—The Summit of Vesuvius in 1767. (From an original drawing.

VESUVIUS IN MODERN TIMES.

At this time we see that inside the crater a series of cones had been built up one within the other from which lava issued, filling the bottom of the crater and finding its way through a breach in its walls, down the side of the cone. It is evident that the ejected materials falling on the sides of the innermost cone would tend to enlarge the latter till its sides became confluent with the cone surrounding it, and if this action went on long enough, the crater would be entirely filled up and a perfect cone with only a small aperture at the top would be produced. But from time to time, grand and paroxysmal outbursts have occurred at Vesuvius, which have truncated the cone, and sometimes formed great, cup-shaped cavities, reaching almost to its base, like that shown in [fig. 13].

In 1767 the crater of Vesuvius, as shown in [fig. 15], contained a single small cone in a state of constant spasmodic outburst, like that of Stromboli.

Fig. 16.—Summit of Vesuvius in 1848.

In 1843, we find that the crater of Vesuvius contained three such small cones arranged in a line along its bottom as depicted in [fig. 16].

These drawings of the summit of Vesuvius give a fair notion of the changes which have been continually going on there during the whole of the historical period. Ever and anon a grand outburst, like that of 1822, has produced a vast and deep crater such as is represented in [fig. 13], and then a long continuance of quiet and regular ejections has built up within the crater small cones like those shown in figs. [14], [15] and [16], till at last the great crater has been completely filled up, and the cone reconstructed.

Fig. 17.—Outlines of Vesuvius, showing its Form at different periods of its history.

CHANGES IN OUTLINE OF VESUVIUS.

In the series of outlines in [fig. 17], we have endeavoured to illustrate the succession of changes which has taken place in Vesuvius during historical times. In the year 79 one side of the crater-wall of the vast mountain-mass was blown away. Subsequent ejections built up the present cone of Vesuvius within the great encircling crater-wall of Somma, and the form of this cone and the crater at its summit have been undergoing continual changes during the successive eruptions of eighteen centuries.

What its future history may be we can only conjecture from analogy. It may be that a long continuance of eruptions of moderate energy may gradually raise the central cone till its sides are confluent with those of the original mountain; or it may be that some violent paroxysm will entirely destroy the modern cone, reducing the mountain to the condition in which it was after the great outburst of 79. On the other hand, if the volcanic forces under Vesuvius are gradually becoming extinct (but of this we have certainly no evidence at present), the mountain may gradually sink into a state of quiescence, retaining its existing form.

The series of changes in the shape of Vesuvius, which are proved by documentary evidence to have been going on during the last 2,000 years, probably find their parallel in all active volcanoes. In all of these, as we shall hereafter show, the activity of the vents undergoes great vicissitudes. Periods of continuous moderate activity alternate with short and violent paroxysmal outbursts and intervals of complete rest, which may in some cases last for hundreds or even thousands of years. During the periods of continuous moderate activity, the crater of the volcano is slowly filled up by the growth of smaller cones within it; and the height of the mountain is raised. By the terrible paroxysmal outbursts the mountain is often completely gutted and its summit blown away; but the materials thus removed from the top and centre of the mass are for the most part spread over its aides, so that its bulk and the area of its base are thereby increased. During the intervals of rest, the sides of the mountain which are so largely composed of loose and pulverulent materials are washed downwards by rains and driven about by winds. Thus all volcanoes in a state of activity are continually growing in size every ejection, except in the case of those where the materials are in the finest state of subdivision, adding to their bulk; the area of their bases being increased during paroxysmal outbursts, and their height during long-continued moderate eruptions.

DEVIATIONS FROM CONICAL FORM.

We have pointed out that the conical form of volcanic mountains is due to the slipping of the falling materials over one another till they attain the angle at which they can rest. There are, however, some deviations from this regular conical form of volcanoes which it may be well to refer to.

The quantity of rain which falls during volcanic eruptions is often enormous, owing to the condensation of the great volumes of steam emitted from the vent. Consequently the falling lapilli and dust often descend upon the mountain, not in a dry state but in the condition of a muddy paste. Many volcanic mountains have evidently been built up by the flow of successive masses of such muddy paste over their surfaces. Some volcanic materials when mixed with water have the property of rapidly 'setting' like concrete. The ancient Romans and modern Italians, well acquainted with this property of certain kinds of volcanic dust and lapilli, have in all ages employed this 'puzzolana,' as it is called, as mortar for building. The volcanic muds have often set in their natural positions, so as to form a rock, which, though light and porous, is of tolerably firm consistency. To this kind of rock, of which Naples and many other cities are built, the name of 'tuff' or 'tufa' is applied. A similar material is known in Northern Germany as 'trass.'

The cause of the 'setting' of puzzolana and tufa is that rain-water containing a small proportion of carbonic acid acts on the lime in the volcanic fragments, and these become cemented together by the carbonate of lime and the free silica, which are thus produced in the mass.

When a strong wind is blowing during a volcanic outburst, the materials may be driven to one side of the vent, and accumulate there more rapidly than on the other. Thus lop-sided cones are formed, such as may frequently be observed in some volcanic districts. In areas where constant currents of air, like the trade-winds, prevail, all the scoria-cones of the district may thus be found to be unequally developed on opposite sides, being lowest on those from which the prevalent winds blow, and highest on the sides towards which these winds blow.

ANGLE OF SLOPE IN VOLCANIC CONES.

The examination of any careful drawing, or better still of the photograph, of a volcanic cone, will prove that the profile of such cones is not formed by straight lines, but by curves often of a delicate and beautiful character. The delineations of the sacred volcano of Fusiyama, which are so constantly found in the productions of Japanese artists, must have familiarised everyone with the elegant curved lines exhibited by the profiles of volcanoes. The upper slope of the mountain is comparatively steep, often exhibiting angles of 30° to 35°, but this steepness of slope gradually diminishes, till it eventually merges in the surrounding plains. The cause of this elegant form assumed by most volcanic mountains is probably two-fold. In the first place we have to remember that the materials falling upon the flanks of the mountain differ in size and shape, and some will rest on a steeper slope than others. Thus, while some of the materials remain on the upper part of the mountains, others are rolling outwards and downwards. Hence we find that those cones which are composed of uniform materials have straight sides. But in some cases, we shall see hereafter, there has certainly been a central subsidence of the mountain mass, and it is this subsidence which has probably given rise to the curvature of its flanks.

We have hitherto considered only the methods by which the froth or foam, which accumulates on the surface of fluid lava, is dispersed. But in many cases not only is this scum of the lava ejected from the volcanic vent by the escaping steam, but the fluid lava itself is extruded forcibly, and often in enormous quantities.

The lava in a volcanic vent is always in a highly heated, usually incandescent, condition. Seen by night, its freshly exposed surface is glowing red, sometimes apparently white-hot. But by exposure to the atmosphere the surface is rapidly chilled, appearing dull red by night, and black by day. Many persons are surprised to find that a flowing stream of lava presents the appearance of a great mass of rough cinders, rolling along with a rattling sound, owing to the striking of the clinker-like fragments against each other. When viewed by night, the gleaming, red light between these rough, cindery masses betrays the presence of incandescent materials below the chilled surface of the lava-stream.

No fact in connection with lavas is more striking than the varying degrees of liquidity presented by them in different cases. While some lava-streams seem to resemble rivers, the material flowing rapidly along, filling every channel in its course, and deluging the whole country around, others would be more fitly compared to glaciers, creeping along at so slow a rate that the fact of their movement can only be demonstrated by the most careful observation. Even when falling over a precipice such lavas, owing to their imperfect liquidity, form heavy, pendent masses like a 'guttering' candle, as is shown by [fig. 18], which is taken from a drawing kindly furnished to me by Capt. S. P. Oliver, R.A. The causes of these differences in the rate of motion of lava-streams we must proceed to consider.

Fig. 18.—Cascade of Lava tumbling over a cliff in the Island of Bourbon.

TEMPERATURE OF LAVA-STREAMS.

There can be no doubt that the temperature of lavas varies greatly in different cases. This is shown by the fact that while some lavas are in a state of complete fusion, similar to that of the slags of furnaces, and like the latter, such lavas on cooling form a glassy mass, others consist of a liquid magma in which a larger or smaller number of crystals are found floating. In these latter cases the temperature of the magma must be below the fusing-point of the minerals which exist in a crystalline condition in its midst. It has indeed been suggested that the whole of the crystals in lavas are formed during the cooling down of a completely fused mass; but no one can imagine that the enclosed crystals of quartz, felspar, leucite, olivine, &c., have been so formed, such crystals being sometimes more than an inch in diameter. The microscopic examination of lavas usually enables us to discriminate between those complete crystals which have been formed at great depths and carried up to the surface, and the minute crystalline particles and microliths which have been developed in the glassy mass during cooling. Crystals of the former class, indeed, exhibit abundant evidence, in their liquid cavities and other peculiarities, that they have not been formed by simple cooling from a state of fusion, but under the combined action of heat, the presence of water and various gases, and intense pressure.

As we have already seen, the different lavas vary greatly in their degrees of fusibility. The basic lavas, containing a low percentage of silica, are much more fusible than the acid lavas, which contain a high percentage of silica. When the basic lavas are reduced to a complete state of fusion their liquidity is sometimes very perfect, as is the case at Kilauea in Hawaii, where the lava is thrown up into jets and fountains, falling in minute drops, and being drawn out into fine glassy threads. On the other hand, the less fusible acid lavas appear to be usually only reduced to the viscous or pasty condition, which artificial glasses assume long before their complete fusion. Of this fact I have found many proofs in the Lipari Islands, where such glassy, acid lavas abound. In [fig. 6] (page 43) a lava-stream is represented on the side of the cone of Vulcano.

IMPERFECTLY FLUID LAVAS.

This lava is an obsidian—that is to say, it is of the add type and completely glassy—but its liquidity must have been very imperfect, seeing that the stream has come to a standstill before reaching the bottom of a steep slope of about 35°. In [fig. 19] there is given a side view of the same stream of obsidian, from which it will be seen that it has flowed slowly down a steep slope and heaped itself up at the bottom, as its fluidity was not complete enough to enable it to move on a slighter incline. An examination of the interior of such imperfectly fluid lavas affords fresh proofs of the slow and tortuous movements of the mass. Everywhere we find that the bands of crystallites and sphærulites are, by the movement of the mass, folded and crumpled and puckered in the most remarkable manner, as is illustrated in figs. [20] and [21]. Similar appearances occur again and again among the vitreous and semi-vitreous acid lavas of Hungary.

Fig. 19.—Lava-stream (obsidian) in the Island of Vulcano showing the imperfect liquidity of the mass.

Fig. 20.—Interior of a Rhyolitic Lava-stream in the Island of Lipari, showing broad sigmoidal folds produced by the slow movements of the mass.

Fig. 21.—Interior of a Rhyolitic Lava-stream in the Island of Lipari, showing the complicated crumplings and puckerings produced by the slow movements of the mass.

RATE OF MOVEMENT OF VESUVIAN LAVAS.