Near the high-road which passes between the towns of Eger and Franzenbad in Bohemia, there rises a small hill known as the Kammerbühl (see fig. 33), which has attracted to itself an amount of interest and attention quite out of proportion to its magnitude or importance. During the latter part of the last century and the earlier years of the present one, the fiercest controversies were waged between the partisans of rival schools of cosmogony over this insignificant hill; some maintaining that it originated in the combustion of a bed of coal, others that its materials were entirely formed by some kind of 'aqueous precipitation,' and others again that the hill was the relic of a small volcanic cone.

Among those who took a very active part in this controversy was the poet Goethe, who stoutly maintained the volcanic origin of the Kammerbühl, styling it 'a pocket edition of a volcano.' To Goethe belongs the merit of having suggested a Very simple method by which the controversies concerning this hill might be set at rest: he proposed that a series of excavations should be undertaken around the hill, and a tunnel driven right under its centre.

The poet's friend, Count Caspar von Sternberg, determined to put this project into execution. This series of excavations, which was completed in 1837, has for ever set at rest all doubts as to the volcanic origin of the Kammerbühl. A plug of basalt was found filling the centre of the mass, and connected with a small lava-stream flowing down the side of the hill; while the bulk of the hill was shown to be composed of volcanic scoriæ and lapilli. The section fig. 34 will illustrate the structure of the hill as revealed by these interesting excavations.

It can of course very seldom happen that actual mining operations, like those undertaken in the case of the Kammerbühl, will be resorted to in order to determine the structure of volcanic mountains. Geologists have usually to avail themselves of less direct, but by no means less certain, methods than that of making artificial excavations in order to investigate the earth's crust. Fortunately it happens that what we cannot accomplish ourselves, nature does for us. The action which we call 'denudation' serves as a scalpel to dissect volcanic mountains for us, and to expose their inner recesses to our view. Many portions of the earth's surface are complete museums crowded with volcanic 'subjects,' exhibiting every stage of the process of dissection. In some, rains and winds have stripped off the loose covering of cinders and dust, and exposed the harder and more solid parts—the skeleton of the mountain. In others, the work of destruction has proceeded still further, and slowly wearing rivers or the waves of the sea may have cut perfect, vertical sections of the mountain-mass. Sometimes the removal of the materials of the volcanic mountain has gone on to such an extent that its base and ground-plan are fully exposed. It only requires the necessary skill in piecing together our observations on these dissected volcanoes, in order to arrive at just views concerning the 'comparative anatomy' of volcanoes. As the knowledge of the structure of animals remained in the most rudimentary condition until the practice of dissection was commenced, so our knowledge of volcanoes was likewise exceedingly imperfect till geologists availed themselves of the opportunities afforded to them of studying naturally dissected volcanic mountains.

In some cases we may find that the sea has encroached on the base of a volcanic hill, till one half of it has been washed away, and the structure of the mass to its very centre is exposed to our view. Thus in fig. 6 (page 43), it will be seen that there lies in front of Vulcano a peninsula called Vulcanello, consisting of three volcanic cones, united at their base, with the lava-streams which have flowed from them. One half of the cone on the left-hand side of the picture has been completely washed away by the sea, and a perfect section of the internal structure of the cone is exposed. The appearances presented in this section are shown in the sketch, fig. 35. Some portions of the face of this section are concealed by the heaps of fragments which have fallen from it, but enough is visible to convince us that three kinds of structures go to make up the cone. In the first place, we have the loose scoriæ and lapilli, which in falling through the air have arranged themselves in tolerably regular layers upon the sides of the cone.

In the second place, we have lava-streams which have been ejected from the crater or from fissures on the flanks of the cone, and flowed down its sides. And thirdly, we find masses of lava filling up cracks in the cone; these latter are called 'dykes.' Of these three kinds of structures most volcanic mountains are built up, but in different cases the part played by these several elements may be very unequal. Sometimes volcanoes consist entirely of fragmentary materials, at others they are made up of lavas only, while in the majority of cases they have been formed by alternations of fragmentary and fluid ejections, the whole being bound together by dykes, which are masses of lava injected into the cracks formed from time to time in the sides of the growing cone.

If we direct our attention in the first place to the fragmentary ejections, we shall find that they affect a very marked and peculiar arrangement, which is best exhibited in those volcanic cones composed entirely of such materials.

Everyone who examines volcanoes for the first time will probably be struck by the regular stratification of materials of which they are composed. Thus the tuffs covering the city of Pompeii are found to consist of numerous thin layers of lapilli and volcanic dust, perfectly distinct from one another, and assuming even the arrangement which we usually regard as characteristic of materials that have been deposited from a state of suspension in water. The fragmentary materials in falling through the air are sorted, the finer particles being carried farther from the vent than the larger and heavier ones. The force of different volcanic outbursts also varies greatly, and sometimes materials of different character are thrown out during successive ejections. These facts will be illustrated by fig. 36, which is a drawing of a section exposed in a quarry opened in the side of the Kammerbühl. In this section we see that the falling scoriæ have been arranged in rudely parallel beds, but the regular deposition of these has been interrupted by the ejection of masses of burnt slate torn from the side of the vent, probably during some more than usually violent paroxysm of the volcano. In those volcanoes which are built up of tuffs and materials which have fallen in the condition of a muddy paste, the perfect stratification of the mass is often very striking indeed, and large cones are found built up of thin uniformly-spread layers of more or less finely-divided materials, disposed in parallel succession. Such finely-stratified tuff-cones abound in the district of the Campi Phlegræi.

If, in consequence of any subterranean movements, fissures are produced in the sides of the cones formed of fragmentary materials, these often become gradually filled with loose fragments from the sides of the fissure, and in this manner 'pseudo-dykes' are formed. An example of such pseudo-dykes is represented in fig. 36, where the beds composing the volcanic cone of the Kammerbühl are seen to have been broken across or faulted, and the fissures produced in the mass have been gradually filled with loose fragments.

It is not difficult to imitate, on a small scale, the conditions which exist at those volcanic vents from which only fragmentary materials are ejected. If we take a board having a hole in its centre, into which a pipe is inserted conveying a strong air-blast, we shall, by introducing some light material like bran or sawdust into this pipe cause an ejection of fragments, which will, when the board is placed horizontally, fall around the orifice of the pipe and accumulate there in a conical heap (fig. 37). It will be found necessary, as was shown by Mr. Woodward, who performed the experiment before the Physical Society, to adopt some contrivance, such as a screw, for forcing the material into the air-pipe. If we alternately introduce materials of different colours, like mahogany- and deal-sawdust into the pipe, these materials will be arranged in layers which can be easily recognised, and the mode of accumulation of the mass will be evident. By means of a sheet of tin or cardboard we may divide this miniature volcanic cone vertically into two portions, and if we sweep one of these away the internal structure of the other half will be clearly displayed before our eyes.

In this way we shall find that the conical heap of sawdust with the hole in its centre has a very peculiar and definite arrangement of its materials. It is made up of a number of layers each of which slopes in opposite directions, towards the centre of ejection and away from that centre. These layers are thickest along the line of the circle where the change in slope takes place, and they thin away in the direction of the two opposite slopes.

The cause of this peculiar arrangement of the materials is evident. The sawdust thrown up by the air blast descends in a shower and tends to accumulate in a circular heap around the orifice, the area of this circular heap being determined by the force of the blast. Within this circular area, however, the quantity of falling fragments is not everywhere the same; along a circle surrounding the vent at a certain distance, the maximum number of falling fragments will be found to descend, and here the thickest deposit will take place. As this goes on, a circular ridge will be formed, with slopes towards and away from the centre of injection. As the ridge increases in height, the materials will tend to roll down either one slope or the other, and gradually a structure of the form shown in the figure will be piled up. The materials sliding down the outer slope will tend to increase the area of the base of the cone, while those which find their way down the inner slope will fall into the vent to be again ejected.

Volcanic cones composed of scoriæ, dust, &c. are found to have exactly the same internal structure as is exhibited by the miniature cone of sawdust. The more or less regular layers of which they are made up dip in opposite directions, away from and towards the vent, and thin out in the direction of their dip (see fig. 38). In small cones the crater or central cavity is of considerable size in proportion to the whole mass, but as the cone grows upwards and outwards, the dimensions of the crater remain the same, while the area of the base and the height of the cone are continually increasing. This is the normal structure of volcanic cones formed of fragmentary materials, though, as we shall hereafter show, many irregularities are often produced by local and temporary causes.

In some cases the central vent of a volcanic scoria-cone may be filled up by subsequent ejections. A beautiful example of this kind was observed by Abich, in the case of a small cone formed within the crater of Vesuvius in 1835, and is represented in fig. 39.

Many cones formed in the first instance of scoriæ, tuff, and pumice may give rise to streams of lava, before the vent which they surround sinks into a state of quiescence. In these cases, the liquid lava in the vent gives off such quantities of steam that masses of froth or scoriæ are formed, which are ejected and accumulate around the orifice. When the force of the explosive action is exhausted, the lava rises bodily in the crater, which it more or less completely fills. But, eventually, the weaker side of the crater-wall yields beneath the pressure of the liquid mass, and this part of the crater and cone is swept away before the advancing lava-stream. Examples of such 'breached cones' abound in Auvergne and many other volcanic districts (see fig. 40). A beautiful example of a cone formed of pumice, which has been breached by the outflow of a lava-stream of obsidian, occurs in the Lipari Islands, at the Rocche Rosse. It is this locality which supplies the whole world with pumice (see fig. 41).

It is often surprising to find how volcanic cones composed of loose materials, such as tuffs, scoriæ, or pumice, retain their distinctive forms, and even the sharpness of their outlines, during enormous periods of time. Thus, in the scoria-cones which abound in the Auvergne, and were, in all probability, formed before the historical period, the sharp edges of the craters appear to have suffered scarcely any erosion, and the cones are as perfect in their outlines as though formed but yesterday. It is probable that the facility with which these cindery heaps are penetrated by the rain which falls upon them is the cause why they are not more frequently washed away.

Sometimes, however, scoria-cones are found reduced by atmospheric waste to mere heaps of cinders, in which the position of the crater is indicated only by a slight depression, as in fig. 42.

When but little explosive action takes place at the volcanic vent, and only fluid lava is ejected, mountains are formed differing very greatly in character from the cones composed of fragmentary materials.

If the lavas be of very perfect liquidity, like those erupted in the Sandwich Islands, they flow outwards around the vent to enormous distances. By the accumulation of materials during successive outbursts, a conical mass is built up which has but a slight elevation in proportion to the area of its base. Thus in Hawaii we find great volcanic cones, composed of very fluid lavas, which have a height of nearly 14,000 feet with a diameter of base of seventy miles. In these Hawaiian mountains the slope of the sides rarely exceeds 6° to 8°.

But if, on the other hand, the lavas be of much more viscid consistency, the character of the volcanic cones which are produced by their extrusion will be very different. The outwelling material will tend to accumulate and heap itself up around the vent. By successive ejections the first-formed shell is forced upwards and outwards, and a steep-sided protuberant mass is formed, exhibiting in its interior a marked concentric arrangement. Dr. Ed. Reyer, of Grätz, has devised a very ingenious method for reproducing on a miniature scale the characteristic features of these eruptions of viscid lavas. He takes a quantity of plaster of Paris reduced to a pasty consistence, which he forces through a hole in a board. The plaster accumulates in a great rounded boss about the orifice through which it has been forced. If the plaster have some colouring matter introduced into it, the mass, on being cut across, will exhibit in the disposition of its colour-bands the kind of action which has gone on during its extrusion, fig. 43.

There are many volcanic cones which exhibit clear evidence of having thus been formed by the extrusion of a viscid mass of lava through a volcanic fissure. Among such we may mention the domitic Puys of Auvergne, fig. 44, many andesitic volcanoes in Hungary, the phonolite hills of Bohemia, and the so-called 'mamelons' of the Island of Bourbon. See figs. 45 and 46. When the interior of these masses is exposed by natural or artificial sections, they are all found to exhibit the onion-like structure which occurs in the plaster models.

But while some volcanoes are composed entirely of the fragmentary ejections and others are wholly formed by successive outflows of lava, the majority of volcanoes, especially those of larger dimensions, are built up of alternations of these different kinds of materials.

The structure of these composite cones may be understood by an inspection of the accompanying fig. 47, which shows the appearances presented in a cliff on the coast of the Island of Madeira. We see that the mass is made up of numerous layers of volcanic scoriæ, alternating with sheets of lava. The latter, which are represented in transverse section in the drawing, are seen to thin out on either side, and to vary greatly in breadth. Besides the alternating masses of scoriæ and the lava-sheets, there are seen in the section, bands of a bright-red colour, which are represented in the drawing by black lines. These are layers of soil, or volcanic dust, which, by the passage of a lava-stream over their surface, have been burnt so as to acquire a brick-red colour. These bands of red material, to which the name of 'laterite' has been frequently applied, very commonly occur in sections of composite volcanic cones. Crossing the whole of the horizontally-disposed masses in the section, we find a number of 'dykes,' which are evidently great cracks filled with lava from below. Some of these run vertically through the cliffs, others obliquely. In some cases the lava, rising to fill a dyke, has flowed as a lava-stream at the surface. Last of all, we must call attention to the fact that the section exhibits evidence of great movements having taken place subsequently to the accumulation of the whole of the materials. A great crack has been produced, on one side of which the whole mass has subsided bodily, giving rise to the phenomenon which geologists call a 'fault.'

In the section, fig. 27, p. 104, copied from a drawing of a sea-cliff in the Island of Vulcano, a transverse section of a lava-stream is represented on a somewhat larger scale. The upper and under surface of the lava-stream is seen to have a scoriaceous structure, but the thick central mass is compact, and divided by regular joint-planes. This section also illustrates the fact that, before the lava-stream flowed down the sides of the mountain, a valley had been cut by meteoric agencies on the flanks of the volcano, the dykes which traverse the lower beds of tuff being abruptly truncated.

In mountain ravines, upon the slopes of ancient volcanoes, and in the cliffs of volcanic islands, we are often able to study the way in which these great mountain masses are built up of alternating lava-currents, beds of volcanic agglomerate, scoriæ, tuff and dust, and intersecting dykes. In fig. 48, the features above described are illustrated by a section in the sides of the great volcano of Mont Dore.

In figs. 49, 50, 51, and 52, we have given drawings of portions of the sea-cliffs in several of the Ponza Islands, a small volcanic group off the Italian coast.

Fig, 53 represents a cliff-section in the island of Salina, one of the Liparis, exhibiting evidence that a series of volcanic agglomerates traversed by dykes of Andesite have been denuded and covered by a recent stratified deposit.

In the formation of these great composite cones, a minor but by no means insignificant part is played by the dykes, or lava-filled fissures, which are seen traversing the mass in all directions. That dyke-fissures often reach the surface of a volcanic cone, and that the material which injects them then issues as a lava-stream, is illustrated by fig. 54. The formation of these cracks in a volcanic cone, and their injection by liquid lava, must of course distend the mountainous mass and increase its volume. If we visit the great crater-walls of Somma in Vesuvius, and of the Val del Bove in Etna, we shall find that the dykes are so numerous that they make up a considerable portion of the mass. When the loose scoriæ and tuffs are removed by denudation, these hard dykes often stand up prominently like great walls, as represented in fig. 55. Even in such cases as these, however, it is doubtful whether the bulk of all the dykes put together exceeds one-tenth of that of the lavas and fragmentary materials.

Hence we are led by an examination of the internal structure of volcanic mountains to conclude that scoriæ- and tuff-cones, and cones formed of very liquid lavas, increase by an exogenous mode of growth, all new materials being added to them from without; in the cones formed of very viscid lavas, on the other hand, the growth is endogenous, taking place by successive accretions within it. The composite cones owe their origin to both the exogenous and the endogenous modes of growth, but in a much greater degree to the former than the latter. The layers of scoriæ, tuff, and dust, and the successive lava-streams are added to the mass from without, and the lava forming the dykes from within it.

There are doubtless cases in which, when a tuff-cone is formed, a mass of very viscid lavas is extruded into its interior, and the mass is distended like a gigantic bubble. But inasmuch as the very viscid lavas do not appear to give rise to scoriæ to anything like the same extent as the more liquid kinds, such 'cupolas,' as they have been called by some German geologists, are probably not very numerous, and may be regarded as constituting the exception rather than the rule. The idea which was formerly entertained by some geologists that all great volcanic mountains were formed of masses originally deposited in a horizontal position, and subsequently blown up into a conical form, has been effectually disposed of by the observations of Lyell and Scrope.

The condition of the great fluid masses which underlie volcanic vents is another point on which much light has been thrown by the study of naturally-dissected volcanoes. In some cases, as was shown by Hochstetter during his admirable researches among the New Zealand volcanoes, the rising lavas form a great chamber for themselves in the midst of a volcanic cinder-cone, taking the place of loose materials which are re-ejected from the vent, or have been re-fused and absorbed into the mass of lava itself. From this central reservoir of lava, eruptions are kept up for some time, but when the volcano sinks into a state of quiescence the lava slowly consolidates. In such slowly solidified masses of lava, very beautiful groups of radiating columns are often exhibited Northern Germany abounds with examples of such basaltic masses, which have once formed the centres of great cinder-cones; but in consequence of the removal of the loose materials and the surrounding strata by denudation, these central reservoirs of the volcanoes have been left standing above the surface, and exhibit the peculiar arrangements of the columns formed in them during the process of cooling.

But in the majority of the more solidly-built composite volcanoes no such liquid reservoir can be formed within the volcanic cone itself. Under these circumstances, the lavas, especially those of more liquid character, tend to force passages for themselves among the rocks through which they are extruded. Wherever a weak point exists, there such lavas will find their way, and as the planes of stratification in sedimentary rocks constitute such weak places, we constantly find sheets of lava thus inserted between beds of aqueous origin. The areas over which these intrusive sheets of rock sometimes extend may be very great, but the more fusible, basic lavas (basalt, &c.) usually form much more widely-spreading sheets than the less fusible, acid lavas. In some cases these great intrusive sheets are found extending to a distance of twenty or thirty miles from the centre at which they were ejected, and they often follow the bedding of the strata with which they are intercalated in so regular a manner, that it is difficult for an observer to believe at first sight that they can have been formed in the way which we have described. A closer examination will generally reveal the fact that while these intrusive lava-sheets retain their parallelism with the strata among which they have been intruded, over considerable areas, yet they sometimes break across, or send offshoots into them, as shown in fig. 56. In all cases, too, the rocks lying above and below such sheets will be found to be more or less baked and altered, and this affords a very convincing evidence of the intrusion of the igneous mass between the strata so altered.

That in the case of most great volcanic mountains, or systems of mountains, vast reservoirs of liquid lava must exist in the earth's crust far below the surface, there can be little room for doubt. Whether such fluid masses are in direct or indirect communication with a great central reservoir, even supposing such to exist, is a totally different question. In many cases the outburst of volcanoes in more or less close proximity has been observed to take place simultaneously, while in others the commencement of the eruption of one volcano has coincided with the lapse into quiescence of another in its vicinity. On the other hand, the remarkable case of the volcanoes of Hawaii seems to indicate that two vents in close proximity may be supplied from perfectly distinct reservoirs of lava. The active craters of Mauna Loa and Kilauea are situated at the heights of 14,000 and 4,000 feet respectively above the sea level; yet the former is sometimes in a state of violent activity, with which the latter shows no signs of sympathy whatever. We shall, in a future chapter, adduce evidence that the liquid lavas in underground reservoirs may undergo various stages of change in the enormous periods of time during which habitual volcanic vents are supplied from them.

We have already shown that the character assumed by a mass of fused material in cooling varies greatly according as the cooling takes place rapidly at the surface or slowly under enormous pressure. In the former case a glassy base is formed containing a greater or smaller number of crystallites or embryo crystals, in the latter the whole rock is converted into a mass of fully-developed crystals.

The lavas which are poured out at the surface consist, as we have seen, of a glassy magma in which a greater or smaller number of crystals are found which have been borne up from below. The great dykes and intrusive sheets consist for the most part of a mass of small or imperfectly developed crystals in which a number of large and perfectly formed crystals are embedded. Such rocks are said to have a 'porphyritic' structure. The rocks formed by the consolidation of the liquid masses in the underground reservoirs are found to be perfectly crystallised, the crystals impressing one another on every side and making up the whole mass to the exclusion of any paste or magma between them. The crystals in those rocks which have consolidated at these vast depths exhibit evidence, in their enclosed watery solutions and liquefied carbonic acid, of the enormous pressures under which they must have been consolidated. The lavas, the more or less porphyritic rocks of the dykes and sheets, and the perfectly crystalline (granitic) rocks of the underground reservoirs pass into one another, however, by the most insensible gradations.

We sometimes find examples of volcanoes which, by the action of denuding forces, have had their very foundations exposed to our view. Such examples occur in the Western Isles of Scotland, in the Euganean Hills near Padua in Northern Italy, and in many other parts of the earth's surface. In these cases we are able to trace the ground-plan of the volcanic pile, and to study the materials which have consolidated deep beneath the surface in the very heart of the mountain.

In studying these 'basal wrecks' of old volcanoes it is always necessary to bear in mind that the appearance and general characters of a volcanic rock may be completely disguised by chemical changes going on within it. It is through want of attention to this fact that so many mistakes were made by the Wernerian school of geologists who declared that they could find no analogy between the basaltic rocks of the globe and the products of active volcanoes, and were hence led to refer the origin of the former to some kind of 'aqueous precipitation.'

Many of the hard and crystalline marbles which are employed as ornamental stones were originally loose masses of shells and corals, as we easily perceive when we examine the polished faces. But these incoherent heaps of organic débris have been converted into a compact and solid rock in consequence of the mass being penetrated by water containing carbonate of lime in solution. Crystals of this substance were deposited in every cavity and interstice of the mass, and thus the accumulation of separate organisms was gradually transformed to a material of great solidity and hardness.

In precisely the same way loose heaps of scoriæ, lapilli, or pumice may, by the passage through them of water containing various substances in solution, have their vesicles filled with crystals, and thus be converted into the hardest and most solid of rock-masses. Similarly the scoriaceous portions of lava-streams have their vesicles filled with crystalline substances deposited from a state of solution, and are thus converted into a solid mass which may at first sight appear to offer but little resemblance to the vesicular materials of recent lava-streams. To these vesicular rocks which have their cavities filled with crystalline substances geologists apply the name of amygdaloids (L. amygdalus, an almond). The cavities in lava-rocks are usually more or less elongated, owing to the movement of the mass while in a still plastic state, and the crystalline materials filling these cavities take the almond-like shape; hence the name.

When the amygdaloids and altered fragmentary ejections of volcanoes are studied microscopically, their true character is at once made manifest. The exposure of faces of these altered volcanic rocks to the weathering influences of the atmosphere, in many cases also causes their true nature to be revealed, the crystalline materials filling the interstices and vesicles of the mass are dissolved away by the rain-water containing carbonic acid, and the rock regains its original cavernous structure and appearance. But this repeated passage of water through volcanic rock-masses may result in the removal of so large a portion of their materials that the remainder crumbles down into the condition of a clay or mud.

In the basal wrecks of volcanoes, of which we have spoken, we usually find only small and fragmentary remains of the great accumulations of loose and scoriaceous materials which originally constituted the bulk of the mountain mass. In the centre of the ground-plan of such a denuded volcano we find great masses of highly crystalline or granitic rock, which evidently occupy vast fissures broken through the sedimentary or other rocks upon which the volcanic pile has been reared. These highly crystalline rocks exhibit, as we have shown, clear evidence of having been consolidated from a state of fusion with extreme slowness and under enormous pressure, but their ultimate chemical composition is identical with that of the lavas which have been ejected from the volcano.

When, as frequently happens, the volcano, after pouring out one kind of lava for a certain period, has changed the nature of its ejections, and given rise to materials of different composition, we find clear evidence of the fact in studying the basal wreck or ground-plan of the volcano. A great intrusive crystalline mass, of the same chemical composition as the first-extruded lava, is found to be rent asunder and penetrated by a similarly crystalline mass having the composition of the lavas of the second period. Thus, in the volcanoes of the Western Isles of Scotland, which are reduced by the action of denudation to this condition of basal wrecks, we find that rhyolites, trachytes, and andesites were ejected during the earlier periods of their history, and basalts during the later periods.

We perceive on studying the ground-plan of these volcanoes that great masses of granite, syenite, and diorite—the crystalline representatives of the first-extruded lavas—are penetrated by intrusions of gabbro—the granitic form of the later-ejected lavas. These features are admirably illustrated by the ruined volcano now constituting the Island of Mull, one of the Inner Hebrides, a plan of which is given in fig. 57, and a section in fig. 58. This volcano probably had a diameter at its base of nearly thirty miles, and a height of from 10,000 to 12,000 feet, but is now reduced to a group of hills few of which exceed 3,000 feet in height.

From these great intrusive masses of highly crystalline rocks there proceed in every direction great spurs or dykes, which are evidently the radiating fissures formed during the outwelling of igneous materials from below, injected by these fluid substances. The rock forming these dykes is often less perfectly crystalline than that which constitutes the centre of the mass, and we may indeed detect among the materials of these dykes examples of every variety of structure, from the perfectly crystalline granite to the more or less glassy substance of lavas. Besides the vertical or oblique dykes we also find horizontal sheets, which, passing from these central masses, have penetrated between the surrounding strata, often, as we have seen, to enormous distances.

For the sake of simplicity, we have spoken of these ground-plans, or basal wrecks of volcanoes, as constituting a flat plain; as a matter of fact, however, the unequal hardness of the materials composing volcanic mountains causes them to assume, under the influence of denuding agencies, a very rugged and uneven surface. The hard crystalline materials filling the central vent stand up as great mountain groups; each large dyke, by the removal of the surrounding softer materials, is left as a huge wall-like mass, while the remnants of lava-streams are seen constituting a number of isolated plateaux.

The great Island of Skye is the basal wreck of another volcano which was also in eruption during Tertiary times; probably, many millions of years ago. This immense volcano had originally a diameter at its base of about thirty miles, and a height of 12,000 to 15,000 feet, and must have been comparable to Etna or Teneriffe in its dimensions. At the present time, there is nothing left of this vast pile but the highly crystalline granites and gabbros filling up the great fissures through which the eruption of igneous materials took place. These, worn by denudation into rounded dome-like masses and wild rugged peaks, constitute the Red Mountains and Coolin Hills of Skye, which rise to the height of more than 3,000 feet above the sea-level. From these great, central masses of crystalline rocks, innumerable radiating dykes may be found rising through the surrounding rock-masses, with isolated patches of the scoriæ and lapilli ejected from the volcano, which have here and there escaped removal by denudation. Along what were the outskirts of this great mountain-mass are found flat-topped hills, built up of lava-streams, only small portions of which have escaped removal by denudation.