That a volcanic vent, when once established, may display intense activity during enormous periods of time, there cannot be the smallest reason for doubting; for the accumulation of materials around some existing volcanic centres must certainly have been going on during many thousands, perhaps millions, of years. To us, whose periods of observation are so circumscribed, it may therefore at first sight appear a hopeless task to trace the 'life-history of a volcano,' to discover the stages of its development, and to indicate the various episodes which have occurred during the long periods it has been in existence. But when it is remembered that we have the opportunity of studying and comparing hundreds of such volcanoes, exhibiting every varying phase of their development, we shall see that such an attempt is by no means so unpromising as it at first sight appears to be. In the present chapter, we shall give an account of the results which have already been obtained by inquiries directed to this object.

There is not the smallest room for doubt that during the past history of our globe, exhibitions of subterranean energy have occurred at many different parts of its surface. There is further evidence that at the several sites where these displays of the volcanic forces have taken place, the succession of the outbursts has run through a regular cycle, gradually increasing in intensity to a maximum, and then as gradually dying away.

A little consideration will show that the first portion of this cycle of events is the one which it is most difficult to examine and study. The products of the earlier and feeble displays of volcanic activity, at any particular centre, are liable to be destroyed, or masked, during the ejection of overwhelming masses of materials in the later stages of its more matured energy. That the feeble displays of volcanic force now exhibited in some localities will gradually increase in intensity in the future, and eventually reach the grandest stage of development, there can be no reason for doubting. But, unfortunately, we are quite unable to discriminate these feeble manifestations, which are the embryonic stages in the development of grand exhibitions of the volcanic forces, from slight outbursts which die away and make no farther sign.

From what has been proved concerning the true nature of volcanic action, however, it is certain that the first step towards the exhibition of such action, at any particular locality, must be the production of an aperture in the earth's crust. Only by means of such an aperture can the vapours, gases, and rocky materials reach the surface, and give rise to the phenomena there displayed. There is reason to believe that all such apertures are really of the nature of fissures, or cracks, which have been opened through the superjacent strata by the efforts of the repressed subterranean forces.

Some recent writers have, it is true, endeavoured to draw a distinction between what they call 'fissure-eruptions,' and eruptions taking place from volcanic cones. But all volcanic outbursts are truly 'fissure-eruptions'—the subterranean materials finding their way to the surface through great cracks, which, in a more or less vertical position, traverse the overlying rock-masses. It is true that in many cases portions of these cracks soon get choked up, while other portions become widened, and the volcanic energy is concentrated at such spots. Thus the materials ejected from these fissures are usually emitted in greatest quantities at one or more points along the fissure, and a single great volcanic vent, or a row of smaller vents, is established upon the line at which the fissure reaches the surface.

We have seen that the amount of explosive action taking place at different volcanic vents varies according to the proportion of imprisoned water contained in the lava. In the cases where there is much explosive action, vast accumulations of scoriæ, lapilli, and dust take place, and cones of great size are built up; but in those cases where the explosive action is small the lavas flow quietly from the vent, and only small scoriæ-cones are thrown up, these being probably soon swept away by the lava-currents themselves or by denuding agencies. But both kinds of eruption have equal claims to be called 'fissure-eruptions.'

In the expansive force of great masses of imprisoned vapour, we have a competent cause for the production of the fissures through which volcanic outbursts take place. Such fissures are found traversing the rocks lying above volcanic foci, and often extending to distances of many miles, or even hundreds of miles, from the centres of activity. Some of these cracks are found to be injected with fused materials from below, others have been more or less completely filled with various minerals that have been volatilized, or carried by superheated waters from the deeper regions of the earth's crust. That many of the cracks thus produced in the superjacent rocks, by the heaving forces of imprisoned vapour seeking to escape, never reached the surface, we have sufficient proof in many mining regions.

If we now transfer our attention from the deeper portions of the earth's crust to the surface, we can well understand how the attempts of the imprisoned vapours to force a passage for themselves through the solid rock-masses would lead to shocks and jars among the latter. Each of these shocks or jars would give rise, in the surrounding portions of the earth's crust, to those vibrations which we know as earthquakes. The close connection between most earthquakes and volcanic phenomena is a fact that does not admit of the smallest doubt; and though it would be rash to define all earthquakes as 'uncompleted efforts to establish a volcano,' yet, in the efforts of the repressed subterranean forces to find a vent by the production of fissures in the overlying rock-masses, we have a cause competent to the production of those shocks which are transmitted to such enormous distances as waves of elastic compression.

We have seen that the production of the fissure upon which the small volcano of Monte Nuovo was thrown up was preceded by a succession of earthquakes, which for a period of over two years terrified the inhabitants of the district, and might have warned them of the coming event. In the same manner, doubtless, the period before the appearance of volcanic phenomena in a new area would be marked by powerful subterranean disturbances within it, due to the efforts of the imprisoned vapours to force for themselves a channel to the surface.

In the case of Monte Nuovo, we have seen that the fissure, when produced, emitted water—at first in a cold, then in a boiling condition—and, eventually, steam and scoriæ. It is probable that through the first cracks which reached the surface, during the heaving of the subterranean forces, water, charged with carbonic acid, flowed abundantly, and that these cold springs, charged with carbonic acid and carbonate of lime, would be succeeded by others which were hot and contained silica in solution. In Hungary, the Western Isles of Scotland, and many other volcanic districts, we find abundant evidence that, before the eruption of lavas in the area, great masses of travertine and siliceous sinter were formed by the action of cold and hot springs.

As the volcanic action became more intense by the more perfect opening of the fissures, the evolution of carbonic add gas would be succeeded by the appearance of sulphurous acid, sulphuretted hydrogen, boracic acid, and hydrochloric acid, which recent studies have shown to be successively emitted from volcanic vents as the temperature within them rises. At last lava or molten rock becomes visible within the fissures, and the ejection of the frothy masses—scoriæ, pumice, lapilli and dust—commences, and this is sometimes succeeded by the outflow of currents of lava.

That volcanoes originate upon lines of fissure in the earth's crust we have the most convincing proofs. Not only have such fissures been seen in actual course of formation at Vesuvius, Etna, and other active volcanoes, but a study of the volcanoes dissected by denudation affords the most convincing evidence of the same fact. The remarkable linear arrangement seen in groups of volcanoes, which is conspicuous to the most superficial observer, confirms this conclusion.

We have described the action going on at Stromboli as typical of that which occurs at all volcanic vents. Stromboli is, however, one among a group of islands all of which are entirely of volcanic origin. The volcanoes of this group of islands, the Æolian or Lipari Islands, are arranged along a series of lines which doubtless mark fissures in the earth's crust. These fissures, as will be seen by the accompanying map (fig. 81), radiate from a centre at which we have proofs of the former existence of a volcano of enormous dimensions. It is a very interesting fact, which the studies of Prof. Suess have established, that the earthquakes which have so often desolated Calabria appear to have originated immediately beneath this great centre of volcanic activity.

When two volcanic cones are thrown up on the same line of fissure, their full development is interfered with, and irregularities in their form and characters are the consequence. In the plan (fig. 82) and the section (fig. 83) an example is given of the results of such a shifting of the centre of eruption along a line of fissure. By the second outburst, one-half of the first-formed cone has been removed, and the second-formed overlaps the first.

Sometimes a number of scoria- or tuff-cones are thrown up in such close proximity to one another along a line of fissure, that they merge into a long irregular heap on the summit of which a number of distinct craters can be traced. An example of this kind was furnished by the line of scoria-cones formed above the fissure which opened on the flanks of Etna in 1865 (see fig. 84).

Even in the case of great composite cones, however, we sometimes find proofs of the centre of eruption having shifted its place along the line of fissure. No better example of this kind could possibly be adduced than that of the Island of Vulcano, with the peninsula of Vulcanello, which is joined to it by a narrow isthmus (see the map, fig. 81, p 192). In fig. 85 we have given an enlarged plan of this island which will make its peculiar structure more intelligible (see also the section given in fig. 77, No. 6, facing p. 178).

The south-eastern part of the island consists of four crater-rings, one half of each of Which has been successively destroyed, through the shifting of the centre of eruption towards the north-west, along the great line of fissure shown in the general map (fig. 81). The last formed of these four crater-rings is the one which is now most complete, and culminates in Monte Saraceno (1581 ft.), a in the plan, the highest point in the island. The older crater-rings have been in part removed by the inroads of the waters of the Mediterranean on the shores of the island. In the centre of the great crater, b, which we have just described, rises the present active cone of Vulcano, 1,266 feet high, and having a crater, c, about 600 yards in diameter and more than 500 feet in depth. From this cone, a great stream of obsidian, e, flowed in the year 1775, and a small crater, d, the Fossa Anticha, has been opened in the side of the cone. The continuation of the same line of fissure is indicated by a ruined tuff-cone, f, known as the Faraglione, and the three scoria-cones of Vulcanello, g, h, which have been thrown up so close to one another as to have their lower portions merged in one common mass, as shown in fig. 86.

Even in volcanoes of the largest dimensions we sometimes find proofs of the centre of eruption having shifted along the line of fissure. Lyell showed that such a change in the position of the central axis of the volcano had taken place in Etna, and the same phenomenon is exhibited in the clearest manner' by some of the ancient volcanoes of the Inner Hebrides, which have been dissected by the denuding forces.

In the case of the Lipari Islands, the fissures along which the volcanic mountains have been thrown up radiate from a common centre, and a similar arrangement can be traced in many volcanic regions, especially those in which a great central volcano has existed. In other cases, however, as in the Campi Phlegræi, the volcanic vents appear to be formed along lines which assume a parallel arrangement, and this doubtless marks the relative position of the original fissures produced in the earth's crust when these volcanoes were formed. In some other cases we find evidences of the existence of a principal fissure from the sides of which smaller cracks originated. These three kinds of arrangements of volcano-producing fissures are equally well illustrated when we study those denuded districts, in which, as we have seen, the ground-plans of volcanic structures are revealed to our view.

There is now good ground for believing that in volcanic vents, at which long-continued eruptive action takes place, the lavas of different chemical composition make their appearance in something like a definite order. It had been remarked by Scrope and other geologists at the beginning of the present century, that in many volcanic areas the acid or trachytic lavas were erupted before the basic or basaltic.

Von Richthofen, by his studies in Hungary and the volcanic districts of the Rocky Mountains, has been able to enunciate a law governing the natural order of succession of volcanic products; and although some exception to this law may be mentioned, it is found to hold good for many other districts than those in which it was first determined.

In a great number of cases it has been found that the first erupted rocks in a volcanic district are those of intermediate composition which are known as andesites. These andesites, which are especially characterised by the nature of their felspar, sometimes contain free quartz and are then known as quartz-andesites or dacites, from their abundance in Transylvania, the old Roman province of Dacia.

Von Richthofen suggests that another class of volcanic rocks, to which he gives the name of 'propylites,' were in every case erupted before the andesites, and in support of his views adduces the fact that in many instances propylites are found underlying andesites. But the propylites are, in chemical composition, identical with the andesites, and like them present some varieties in which quartz occurs, and others in which that mineral is absent. In their microscopic characters the propylites differ from the andesites and dacites only in the fact that the former are more perfectly crystalline in structure, being indeed in many cases quite undistinguishable from the diorites or the plutonic representatives of the andesites. The propylites also contain liquid cavities, which the andesites and dacites as a rule do not, and the former class of rocks, as Prof. Szabo well points out, are usually much altered by the passage of sulphurous and other vapours, in consequence of which they frequently contain valuable metallic ores.

The extrusion of these andesitic lavas is sometimes accompanied, and sometimes preceded or followed, by eruptions of trachytic lavas—that is, of lavas of intermediate composition which have a different kind of felspar from that prevailing in the andesites.

In the final stages of the eruptive action in most volcanic districts the lavas poured forth belong to the classes of the rhyolitic or acid, and the basaltic or basic lavas.

These facts are admirably illustrated in the case of the volcanic district of the Lipari Islands, to which we have had such frequent occasion to refer. The great central volcano of this district, which now in a ruined condition constitutes a number of small islets (see the map, fig. 81, p. 192), is composed of andesitic lavas. The other great volcanoes thrown up along the three radiating lines of fissure are composed of andesitic and trachytic rocks. But all the more recent ejections of the volcanoes of the district have consisted either of rhyolites, as in Lipari and Vulcano, or of basalts, as in Stromboli and Vulcanello.

Von Richthofen and the geologists who most strongly maintain the generalisations which he has made concerning the order of appearance of volcanic products, go much farther than we have ventured to do, and insist that in all volcanic districts a constant and unvarying succession of different kinds of lavas can be made out. It appears to us, however, that the exceptions to the law, as thus precisely stated, are so numerous as to entirely destroy its value.

The generalisation that in most volcanic districts the first ejected lavas belong to the intermediate group of the andesites and trachytes, and that subsequently the acid rhyolites and the basic basalts made their appearance, is one that appears to admit of no doubt, and is found to hold good in nearly all the volcanic regions of the globe which have been attentively studied.

The Tertiary volcanic rocks of our own country, those of North Germany, Hungary, the Euganean Hills, the Lipari Islands, and many other districts in the Old World, together with the widespread volcanic rocks of the Rocky Mountains in the New World, all seem to conform to this general rule.

In connection with this subject, it may be well to refer to the ideas on the composition of volcanic rocks which were enunciated by Bunsen, and the theoretic views based on them by Durocher. Bunsen justly pointed out that all volcanic rocks might be regarded as mixtures in varying proportions of two typical kinds of materials, which he named the 'normal trachytic' and the 'normal pyroxenic' elements respectively. The first of these corresponds very closely in composition with the acid volcanic rocks or rhyolites, and the second with the basic volcanic rocks or basalts. Durocher pointed out that if quantities of these different materials existed in admixture, the higher specific gravity of the basic element would cause it gradually to sink to the bottom, while the acid element would rise to the top. Carrying out this idea still further, he propounded the theory that beneath the earth's solid crust there exist two magmas, the upper consisting of light acid materials, the lower of heavy basic ones; and he supposed that by the varying intensity of the volcanic forces we may have sometimes one or the other magma erupted and sometimes varying mixtures of the two.

The study of volcanic rocks in recent years has not lent much support to the theoretic views of Durocher concerning the existence of two universal magmas beneath the earth's crust; and there are not a few facts which seem quite irreconcilable with such a theory. Thus we find evidence that in the adjacent volcanic districts of Hungary and Bohemia, volcanic action was going on during the whole of the latter part of the Tertiary period. But the products of the contemporaneous volcanic outbursts in adjacent areas were as different in character as can well be imagined. The volcanic rocks all over Hungary present a strong family likeness; the first erupted were trachytes, then followed andesites and dacites in great abundance, and lastly rhyolites and basalts containing felspar. But in Bohemia, the lavas poured out from the volcanoes during the same period were firstly phonolites and then basalts containing nepheline and leucite. It is scarcely possible to imagine that such very different classes of lavas could have been poured out from vents which were in communication with the same reservoirs of igneous rock, and we are driven to conclude that the Hungarian and Bohemian volcanoes were supplied from different sources.

But the undoubted fact that in so many volcanic regions the eruption of andesitic and trachytic rocks, which are of intermediate composition, is followed by the appearance of the differentiated products, rhyolite and basalt, which are of acid and basic composition respectively, lends not a little support to the view that under each volcanic district a reservoir of more or less completely molten rock exists, and that in these reservoirs various changes take place during the long periods of igneous activity. During the earlier period of eruption the heavier and lighter elements of the contents of these subterranean reservoirs appear to be mingled together; but in the later stages of the volcanic history of the district, the lighter or acid elements rise to the top, and the heavier or basic sink to the bottom, and we have separate eruptions of rhyolite and basalt. We even find some traces of this action being carried still further. Among the basalts ejected from the volcanoes of Northern Germany, Bohemia, Styria, Auvergne, and many other regions, we not unfrequently find rounded masses consisting of olivine, enstatite, augite, and other heavier constituents of the rock. These often form the centre of volcanic bombs, and are not improbably portions of a dense mass which may have sunk to the bottom of the reservoirs of basaltic materials.

In consequence of the circumstance that the eruption of lavas of intermediate composition usually precedes that of other varieties, we usually find the central and older portions of great volcanoes to be formed of andesites, trachytes, or phonolites, while the outer and newer portions of the mass are made up of acid or basic lavas. This is strikingly exemplified in the great volcanoes of the Auvergne and the Western Isles of Scotland, in all of which we find that great mountain masses have, in the first instance, been built up by extrusions of lava of the intermediate types, and that through this central core fissures have been opened conveying basic lavas to the surface. From these fissures great numbers of basaltic lava-streams have issued, greatly increasing the height and bulk of the volcanic cones and deluging the country all around.

The lavas of intermediate composition—the andesites, trachytes, and phonolites—possess, as we have already seen, but very imperfect liquidity as they flow from the volcanic vents. Hence we find them either accumulating in great dome-shaped masses above the vent or forming lava-streams which are of great bulk and thickness, but do not flow far from the orifices whence they issue. The more fusible basaltic lavas, on the other hand, spread out evenly on issuing from a vent, and sometimes flow to the distance of many miles from it. This difference in the behaviour of the intermediate and basic lavas is admirably illustrated in the volcanic districts of the Auvergne and the Western Isles of Scotland.

In other cases, like Vesuvius, we find that great volcanic cones of trachytic tuff have been built up, and that these masses of fragmentary trachytic materials have been surrounded and enclosed by the ejection, at a later date, of great outbursts of basaltic lavas. In still other cases, of which Rocca Monfina in Southern Italy constitutes an excellent example, we find that a great crater-ring of trachytic tuffs has been formed in the first instance, and in the midst of this a cone, composed of more basic materials, has been thrown up.

In all these volcanoes we see the tendency towards the eruption of intermediate lavas in the first instance, and of basaltic and acid lavas at a later date. Valuable, however, as are the early generalisations of Scrope, and the more precise law enunciated by Von Richthofen concerning the 'natural order of succession of volcanic products,' we must not forget that there are to be found a considerable number of exceptions to them. There are some volcanic centres from which only one kind of lava has been emitted, and this may be either acid, basic, or intermediate in composition; and on the other hand, there are districts in which various kinds of lava have been ejected from the same vents within a short period of time, in such a way as to defy every attempt to make out anything like a law as to the order of their appearance. Nevertheless the rules which we have indicated appear to hold good in so great a number of cases that they are well worthy of being remembered, and may serve as a basis on which we may reason concerning the nature of the action going on beneath volcanic vents.

From the study of the external appearances of volcanic mountains, combined with investigations of those which have been dissected by denudation, we are able to picture to our minds the series of actions by which the great volcanic mountains of the globe have been slowly and gradually built up.

In the first instance the eruptions appear to have taken place at several points along a line of fissure, but gradually all of these would become choked up except one which became the centre of habitual eruption. From this opening, ejections, firstly of lavas of intermediate composition, and afterwards of basic materials, would take place, until a volcano of considerable dimensions was built up around it. But at last a point would be reached in the piling up of this cone, when the volcanic forces below would be inadequate to the work of raising the liquid lava through the whole length of the continually upward-growing tube of the volcano. Under these circumstances the expansive force of the imprisoned steam would find it easier to rend asunder the sides of the volcanic cone than to force the liquid material to the summit of the mountain. If these fissures reached the surface explosive action would take place, in consequence of the escape of steam from the glowing mass, and scoria-, tuff-, and lava-cones would be formed above the fissure. In this way, as we have already pointed out, the numerous 'parasitic cones' which usually abound on the flanks of the greater volcanic mountains have been formed. The extrusion of these masses of scoriæ and lava on the flanks of the mountain tends, not only to increase the bulk of the mass, but to strengthen and fortify the sides. For by the powerful expansive force at work below, every weak place in the cone is discovered and a fissure produced there; but by the extrusion of material at this fissure, and still more by the consolidation of the lava in the fissure, the weak place is converted into one of exceptional strength.

As the sides of the cone are thus continually repaired and strengthened they are rendered more capable of withstanding the heaving forces acting from below, and these forces can then only find vent for themselves by again raising the liquefied lava to the central orifice of the mountain. Many volcanoes, like Etna, exhibit this alternation of eruptive action from the crater at the summit of the mountain, and from fissures opened upon its flanks, the former tending to raise the height of the volcanic pile, the latter to increase its bulk.

But at last a stage will be reached when the volcanic forces are no longer able either to raise the lava up the long column of the central vent on the one hand, or to rend asunder the strongly-built and well-compacted flanks of the mountain on the other. It is probably under these conditions, for the most part, that the lavas find their way between the masses of surrounding strata and force them asunder in the way that we have already described.

In the case of the more fluid basaltic lavas, as was pointed out so long ago by Macculloch, the liquefied materials may find their way between the strata to enormous distances from the volcanic centre. Such extended flat sheets of igneous rock retain their parallelism with the strata among which they are intruded over large areas, and did not probably produce any marked phenomena at the surface.

But in the case of less fluid lavas, such as those of intermediate or acid composition, for example, the effect would be far otherwise. Such lavas, not flowing readily from the centre of eruption, would tend to form great bulky lenticular masses between the strata which they forced asunder, and, in so doing, could not fail to upheave and fissure the great mountain-mass above. Vast lenticular masses of trachytic rock, thus evidently forced between strata, have been described by Mr. G. K. Gilbert, as occurring in the Henry Mountains of Southern Utah, and by him have been denominated 'laccolites,' or stone-cisterns. Whether the great basaltic sheets, like those described by Macculloch, and those more bulky lenticular reservoirs of rock of which Mr. Gilbert has given us such an admirable account, were in all cases connected with the surface, may well be a matter for doubt. It is quite possible that, in some cases, liquefied masses of rocky materials in seeking to force their way to the surface only succeeded in thus finding a way for themselves between the strata, and their energy was expended before the surface was reached and explosive action took place. But it is an undoubted fact that beneath many of the old volcanoes, of which the internal structure is now revealed to us by the action of denuding forces, great intrusive sheets and laccolites abound; and we cannot doubt that beneath volcanoes now in a state of eruption, or in those which have but recently become extinct, similar structures must be in course of formation.

That great upheaving forces have operated on volcanoes, subsequently to the accumulation of their materials, we have sufficient evidence in the Val del Bove of Etna, the Caldera of Palma, the Corral of Madeira, &c. In all of these cases we find a radial fissure ('barranco') leading into a great crateral hollow; and these radial fissures are of such width and depth that their origin can only be referred to a disruptive force like that which would be exercised by the intrusion of masses of more or less imperfectly fluid material between the subjacent strata. These facts, of course, lend no countenance to the views formerly held by many geologists, both in Germany and France, that the materials of which volcanoes are built up were deposited in an approximately horizontal position, and were subsequently blown up like a gigantic bubble. In Etna, Palma, and Madeira we find abundant proofs that the mass existed as a great volcanic cone before the production of the fissures (barrancos), which we have referred to the force exercised during the intrusion of great igneous masses beneath them.

But besides the horizontally-disposed intrusive sheets and laccolites, great, radiating, vertical fissures are produced by the heaving forces acting beneath those volcanic centres which have been closed up and 'cicatrised' by the exudation from them of subterranean materials. These vertical intrusions, which we call dykes, like the horizontal ones, differ in character, according to the nature of the materials of which they are composed. Dykes of acid and intermediate lava are usually of considerable width, and do not extend to great distances from the centres of eruption. Dykes composed of the more-liquid, basic lavas, on the other hand, may extend to the distance of hundreds of miles from the central vent. The way in which comparatively narrow, basaltic dykes are found running in approximately straight lines for such enormous distances is a very striking fact, and bears the strongest evidence to the heaving and expanding forces at work at volcanic centres, during and subsequently to the extrusion of the igneous products at the surface.

These basaltic dykes occur in such prodigious numbers around some volcanic vents, that the whole of the stratified rocks in the immediate vicinity are broken up by a complete network of them, crossing and interlacing in the most complicated fashion. Farther away from the vents, similar dykes are found in smaller numbers, evidently radiating from the same centre, and sometimes extending to a distance of more than a hundred miles from it. Nowhere can we find more beautiful illustrations of such dykes than in the Western Isles of Scotland. When composed of materials which do not so easily undergo decomposition as the surrounding rocks, they stand up like vast walls; but when, on the other hand, they are more readily acted on by atmospheric moisture than are the rocks which enclose them, they give rise to deep trenches with vertical sides, which render the country almost impassable.

The lava consolidating in these horizontal intrusions (sheets and laccolites), and the vertical intrusions (dykes), is usually more crystalline in structure than the similar materials poured out at the surface. In the same dyke or sheet, when it is of great width, we often find every variation—from a glassy material formed by the rapid cooling of the mass where it is in contact with other rocks, to the perfectly crystalline or granitic varieties which form the centre of the intrusion. It is in these dykes and other intrusions that we find the most convincing evidence of the truth of the conclusions, which we have enunciated in a former chapter, concerning the dependence of the structure of an igneous rock upon the conditions under which it has consolidated. One material is found, under varying conditions, assuming the characters of obsidian, rhyolite, quartz-felsite, or granite; another, under the same set of conditions, taking the form of tachylyte, basalt, dolerite, and gabbro.

That these great intrusive masses, sheets and dykes, in their passage between the sedimentary rocks sometimes find places where the overlying strata are of such thinness or incoherence that the liquefied rocks are able to force a way for themselves to the surface, we have the clearest proof. In some dykes we find the rock in their upper portions losing its compact character and becoming open and scoriaceous, showing that the pressure had been so far diminished as to allow of the imprisoned water flashing into steam.

All round great volcanoes which have become extinct we frequently find series of small volcanic cones, which have evidently been thrown up along the lines where the great lava-filled fissures, which we have been describing, have reached the surface and given rise to explosive action there. The linear arrangement of these small cones, which are thrown up in the plains surrounding vast volcanic mountains that have become extinct, is very striking. The numerous 'puys' of the Auvergne and adjoining volcanic regions of Central France are for the most part small scoria- and lava-cones which were thrown up along great lines of fissure radiating from the immense, central, volcanic mountains of the district, after they had become extinct. These scoria-cones and the small lava-streams which flow from them, as was so well shown by Mr. Scrope, mark the latest efforts of the volcanic forces beneath the district before they finally sank into complete extinction. In the Western Isles of Scotland, as I have elsewhere shown, we can study the formation of these later-formed cones in the plains around extinct volcanic mountains, with the additional advantage of having revealed to us, by the action of the denuding forces, their connection with the great radiating fissures.

It has been shown that the several stages in the decline of each volcanic outburst is marked by the appearance at the vent of certain acid gases. In the same way, after the ejection of solid materials from a volcanic vent has come to an end, certain gaseous substances continue to be evolved; and as the temperature at the vents declines, the nature of the volatile substances emitted from them undergoes a regular series of changes.

M. Fouqué, by a careful series of analyses of the gases which he collected at different gaseous vents, or fumaroles as they are called, in the crater of Vulcano, has been able to define the general relations which appear to exist between the temperature at a volcanic orifice and the volatile substances which issue from it. He found that in fumaroles, in which the temperature exceeded 360° centigrade, and in which in consequence strips of zinc were fused by the stream of issuing gas, the analysis of the products showed sulphurous acid and hydrochloric add to be present in large quantities, and sulphuretted hydrogen and carbonic acid in much smaller proportions. Around these excessively heated fumaroles, the lips of which often appear at night to be red-hot, considerable deposits of sulphide of arsenic, chloride of iron, chloride of ammonium, boracic acid, and sulphur were taking place.

It was found, however, that as the temperature of the vent declined, the emission of the sulphurous acid and hydrochloric acid diminished, and the quantity of sulphuretted hydrogen and carbonic acid mingled with them was proportionately increased.

In the same way it appears to be a universal rule that when a volcanic vent sinks into a condition of temporary quiescence or complete extinction the powerfully acid gases, hydrochloric acid and sulphurous acid, make their appearance in the first instance, and at a later stage these are gradually replaced by sulphuretted hydrogen and carbonic acid.

Of these facts we find a very beautiful illustration in the Campi Phlegræi near Naples. With the exception of Monte Nuovo, the volcano which has most recently been in a state of activity in that district is the Solfatara. From certain apertures in the floor of the crater of the Solfatara there issue continually watery vapours, sulphurous acid, sulphuretted hydrogen, hydrochloric acid, and chloride of ammonium. The action of these substances upon one another, and upon the volcanic rocks through which they pass, gives rise to the formation of certain chemical products which, from a very early period, have been collected on account of their commercial value. The action of these add gases upon the surrounding rocks is very marked; efflorescent deposits of various sulphates and chlorides take place in all the crevices and vesicles of the rock; sulphur and sulphide of arsenic are also formed in considerable quantities; and the trachytic tuffs, deprived of their iron-oxide, alkaline earths and alkalies, which are converted into soluble sulphates and chlorides, are reduced to a white, powdery, siliceous mass. Many volcanoes, which have sunk into a state of quiescence or extinction like the Solfatara of Naples, exhibit the same tendency to give off great quantities of the powerfully-acid gases which act upon the surrounding rocks, and deprive them of their colour and consistency. Such volcanoes are said by geologists to have sunk into the 'solfatara stage.'

At the Lake of Agnano and some other points in the Campi Phlegræi, however, we find fissures from which the less-powerfully acid gases, sulphuretted hydrogen and carbonic acid, issue. These gases as they, are poured forth from the vents are found to be little, if at all, above the temperature of the atmosphere. Sulphuretted hydrogen is an inflammable gas, and in the so-called salses and mud-volcanoes, at which it is ejected in considerable quantities, it not unfrequently takes fire and bums with a conspicuous flame. Carbonic acid on account of its great density tends to accumulate in volcanic fissures and craters rather than to mingle with the surrounding atmosphere. At the so-called Grotto del Cane, beside the Lago Agnano, it is the custom to show the presence of this heavy and suffocating gas by thrusting a dog into it, the poor animal being revived, before life is quite extinct, by pouring cold water over it. At the Büdos Hegy or 'stinking hill' of Transylvania, carbonic acid and sulphuretted hydrogen are emitted in considerable quantities, and it is possible to take a bath of the heavy gas, the head being kept carefully above the constant level of the exhalations.

Although the stories of the ancient Avernian lake, across which no bird could fly without suffocation, and of the Guevo Upas, or Poison Valley of Java, which it has been said no living being can cross, may not improbably be exaggerations of the actual facts, yet there is a basis of truth in them in the existence of old volcanic fissures and craters which evolve the poisonous sulphuretted hydrogen and carbonic acid gases.

Besides the gases which we have already named, and which are the most common at and characteristic of volcanic vents, there are some others which are not unfrequently emitted. First among these we must mention boracic acid, which, though not a remarkably volatile substance, is easily carried along in a fine state of division in a current of steam. At Monte Cerboli and Monte Rotondo in Tuscany, great quantities of steam jets accompanied by sulphuretted hydrogen and boracic acid issue from the rocks, and these jets being directed into artificial basins of water, the boracic acid is condensed and is recovered by evaporation. We have already noticed that boracic add is evolved with the gases at Vulcano and other craters; and the part which this substance plays in volcanic districts is shown by the fact that many of the rocks, filling old subterranean volcanic reservoirs, are found to be greatly altered and to have new minerals developed in their midst through the action upon them of boracic acid.

Ammonia and various compounds of carbon, nitrogen, and hydrogen are among the gases evolved from volcanic vents. In some cases these gases may be produced by the destructive distillation of organic materials in the sedimentary rocks through which volcanic outbursts take place. But it is far from impossible that under the conditions of temperature and pressure which exist at the volcanic foci, direct chemical union may take place between substances, which at the surface appear to be perfectly inert in each other's presence.

When the temperature at volcanic fissures is no longer sufficiently high to cause water to issue in the condition of vapour or steam, as is the case at the 'stufas' which we have described, it comes forth in the liquid state. Water so issuing from old volcanic fissures may vary in its temperature, from the boiling point downwards.