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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

But, although the temperature of lava-streams and the fusibility of their materials may in some cases account for their condition of either perfect liquidity or viscidity, it is clear that in other instances there must be some other cause for this difference. Thus it has been found that at Vesuvius the lavas erupted in modern times have all a striking similarity to one another in chemical composition, in the minerals which they contain, and in their structure. They are all basic lavas, which when examined by the microscope are seen to consist of a more or less glassy magma, in the midst of which numerous crystals of augite, leucite, olivine, magnetite, and other minerals are scattered. Yet nothing can be more strikingly different than the behaviour of the lavas poured out from Vesuvius at various periods. In some cases the lava appears to be in such a perfectly liquid condition that, issuing from the crater, it has been described as rushing down the slope of the cone like a stream of water, and such exceedingly liquid lavas have in some cases flowed to the distance of several miles from the base of the mountain in a very short time. But other Vesuvian lavas have been in such a viscid condition that their rate of movement has been so extremely slow as to be almost imperceptible. Such lava-streams have continued in movement during many years, but the progress has been so slow (often only a few inches in a day) that it could only be proved by means of careful measurements.

If we examine some of these Vesuvian lavas which have exhibited such striking differences in their rate of flow, we shall find that they present equally marked differences in the character of their surfaces. The lava-current of 1858 was a remarkable example of a slow-flowing stream, and its surface, as will be seen in fig. 22, which is taken from a photograph, has a very marked and peculiar character. A tenacious crust seems to have formed on the surface, and by the further motion of the mass this crust or scum has been wrinkled and folded in a very remarkable manner. Sometimes this folded and twisted crust presents a striking resemblance to coils of rope. Precisely similar appearances may be observed on the surface of many artificial slags when they flow from furnaces, and are seen to be due to the same cause, namely, the wrinkling up of the chilled surface-crust by the movement of the liquid mass below. Lavas which present this appearance are frequently called 'ropy lavas'; an admirable example of them is afforded in the lava-cascade of the Island of Bourbon represented in fig. 18 (page 93).

But lavas in which the rate of flow has been very rapid, exhibit quite a different kind of surface to that of the ropy lavas. The Vesuvian lava-stream of 1872 was remarkable for the rapidity of its flow, and its surface presents a remarkable contrast to that of the slow-moving lava of 1858. The surface of the lava-current of 1872 is covered with rough cindery masses, often of enormous dimensions, and it is exceedingly difficult to traverse it, as the ragged projecting fragments tear the boots and lacerate the skin. The appearance presented by this lava-stream is illustrated by fig. 23, which is also taken from a photograph.