Now on looking at a photographic picture of the moon's surface (Fig. 38), we observe that there are enormous dark spaces, irregular in outline, but more or less approaching the circular form, surrounded by steep and precipitous declivities, but with sides sloping outwards. These were supposed at one time to be seas; and they retain the name, though it is universally admitted that they contain no water. Some of these hollows are four English miles in depth. The largest of these, situated near the north pole of the moon, is called Mare Imbrium; next to it is Mare Serenitatis; next, Mare Tranquilitatis, with several others.[6] Mare Imbrium is of great depth, and from its floor rise several conical mountains with circular craters, the largest of which, Archimedes, is fifty miles in diameter; its vast smooth interior being divided into seven distinct zones running east and west. There is no central mountain or other obvious internal sign of former volcanic activity, but its irregular wall rises into abrupt towers, and is marked outside by decided terraces.[7]

The Mare Imbrium is bounded along the east by a range of mountains called the Apennines, and towards the north by another range called the Alps; while a third range, that of the Caucasus, strikes northward from the junction of the two former ranges. Several circular or oval craters are situated on, and near to, the crest of these ridges.

But the greater part of the moon's hemisphere is dotted over by almost innumerable circular crater-like hollows; sometimes conspicuously surmounting lofty conical mountains, at other times only sinking below the general outer surface of the lunar sphere. On approaching the margin, these circular hollows appear oval in shape owing to their position on the sphere; and the general aspect of those that are visible leads to the conclusion that there are large numbers of smaller craters too small to be seen by the most powerful telescopes. These cones and craters are the most characteristic objects on the whole of the visible surface, and when highly magnified present very rugged outlines, suggestive of slag, or lava, which has consolidated on cooling, as in the case of most solidified lava-streams on our earth.[8] One of the most remarkable of these crateriform mountains is that named Copernicus, situated in a line with the southern prolongation of the Apennines. Of this mountain Sir R. Ball says: "It is particularly well known through Sir John Herschel's drawing, so beautifully reproduced in the many editions of the Outlines of Astronomy. The region to the west is dotted over with innumerable minute craterlets. It has a central, many-peaked mountain about 2,400 feet in height. There is good reason to believe that the terracing shown in its interior is mainly due to the repeated alternate rise, partial congealation and retreat of a vast sea of lava. At full moon it is surrounded by radiating streaks."[9] The view regarding the structure of Copernicus here expressed is of importance, as it is probably applicable to all the craters of our satellite.

"When the moon is five or six days old," says Sir Robert Ball, "a beautiful group of three craters will be readily found on the boundary line between night and day. These are Catharina, Cyrillus, and Theophilus. Catharina is the most southerly of the group, and is more than 16,000 feet deep and connected to Cyrillus by a wide valley; but between Cyrillus and Theophilus there is no such connection. Indeed Cyrillus looks as if its huge surrounding ramparts, as high as Mont Blanc, had been completely finished when the volcanic forces commenced the formation of Theophilus, the rampart of which encroaches considerably on its older neighbour. Theophilus stands as a well-defined round crater, about 64 miles in diameter, with an internal depth of 14,000 to 18,000 feet, and a beautiful central group of mountains, one-third of that height, on its floor. This proves that the last eruptive efforts in this part of the moon fully equalled in intensity those that had preceded them. Although Theophilus is on the whole the deepest crater we can see in the moon, it has received little or no deformation by secondary eruptions."

But perhaps the most remarkable object on the whole hemisphere of the moon is "the majestic Tycho," which rises from the surface near the south pole, and at a distance of about 1/6th of the diameter of the sphere from its margin. Its depth is stated by Ball to be 17,000 feet, and its diameter 50 miles. But its special distinction amongst the other volcanic craters lies in the streaks of light which radiate from it in all directions for hundreds and even thousands of miles, stretching with superb indifference across vast plains, into the deepest craters, and over the highest opposing ridges. When the sun rises on Tycho these streaks are invisible, but as soon as it has reached a height of 25° to 30° above the horizon, the rays emerge from their obscurity, and gradually increase in brightness until full moon, when they become the most conspicuous objects on her surface. As yet no satisfactory explanation has been given of the origin of these illuminated rays,[10] but I may be permitted to add that their form and mode of occurrence are eminently suggestive of gaseous exhalations from the volcano illumined by the sun's rays; and owing to the absence of an atmosphere, spreading themselves out in all directions and becoming more and more attenuated until they cease to be visible.

The above account will probably suffice to give the reader a general idea of the features and inferential structure of the moon's surface. That she was once a molten mass is inferred from her globular form; but, according to G. F. Chambers, the most delicate measurements indicate no compression at the poles.[11] That her surface has cooled and become rigid is also a necessary inference; though Sir J. Herschel considered that the surface still retains a temperature possibly exceeding that of boiling water.[12] However this may be, it is pretty certain that whatever changes may occur upon her surface are not due to present volcanic action, all evidence of such action being admittedly absent. If, when the earth and moon parted company, their respective temperatures were equal, the moon being so much the smaller of the two would have cooled more rapidly, and the surface may have been covered by a rigid crust when as yet that of the earth may have been molten from heat. Hence the rigidity of the moon's surface may date back to an immensely distant period, but she may still retain a high temperature within this crust. Having arrived at this stage of our narrative, we are in a position to consider by what means, and under what conditions, the cones and craters which diversify the lunar surface have been developed.

In doing so it may be desirable, in the first place, to determine what form of crater on our earth's surface those of the moon do not represent; and we are guided in our inquiry by the consideration of the absence of water on the lunar surface. Now there are large numbers of crateriform mountains on our globe in the formation of which water has played an important, indeed essential, part. As we have already seen, water, though not the ultimate cause of volcanic eruptions, has been the chief agent, when in the form of steam at high pressure, in producing the explosions which accompany these eruptions, and in tearing up and hurling into the air the masses of rock, scoriæ, and ashes, which are piled around the vents of eruption in the form of craters during periods of activity. To this class of craters those of Etna, Vesuvius, and Auvergne belong. These mountains and conical hills (the domes excepted) are all built up of accumulations of fragmental material, with occasional sheets and dykes of lava intervening; and where eruptions have taken place in recent times, observation has shown that they are accompanied by outbursts of vast quantities of aqueous vapour, which has been the chief agent (along with various gases) in piling up the circular walls of the crater.

It has also been shown that in many instances these crater-walls have been breached on one side, and that streams of molten lava which once occupied the cup to a greater or less height, have poured down the mountain side. Hence the form or outline of many of these fragmental craters is crescent-shaped. Such breached craters are to be found in all parts of the world, and are not confined to any one district, or even continent, so that they may be considered as characteristic of the class of volcanic crater-cones to which I am now referring. In the case of the moon, however, we fail to observe any decided instances of breached craters, with lava-streams, such as those I have described.[13] In nearly all cases the ramparts appear to extend continuously round the enclosed depression, solid and unbroken; or at least with no large gap occupying a very considerable section of the circumference. (See Fig. 38.) Hence we are led to suspect that there is some essential distinction between the craters on the surface of the moon and the greater number of those on the surface of our earth.

It is scarcely necessary to add that the volcanic mountains of the moon offer no resemblance whatever to the dome-shaped volcanic mountains of our globe. If it were otherwise, the lunar mountains would appear as simple luminous points rising from a dark floor, over which they would cast a conical shadow. But the form of the lunar volcanic mountains is essentially different; as already observed, they consist in general of a circular rampart enclosing a depressed floor, sometimes terraced as in the case of Copernicus, from which rise one or more conical mountains, which are in effect the later vents of eruption.

In our search, therefore, for analogous forms on our own earth, we must leave out the craters and domes of the type furnished by the European volcanoes and their representatives abroad, and have recourse to others of a different type. Is there then, we may ask, any type of volcanic mountain on our globe comparable with those on the moon? In all probability there is.

If the reader will turn to the description of the volcanoes of the Hawaiian group in the Pacific, especially that of Mauna Loa, as given by Professor Dana and others, and compare it with that of Copernicus, he will find that in both cases we have a circular rampart of solid lava enclosing a vast plain of the same material from which rise one or more lava-cones. The interiors in both cases are terraced. So that, allowing for differences in magnitude, it would seem that there is no essential distinction between lunar craters and terrestrial craters of the type of Mauna Loa. Dana calls these Hawaiian volcanoes "basaltic," basalt being the prevalent material of which they are formed. Those of the moon may be composed of similar material, or otherwise; but in either case we may suppose they are built up of lava, erupted from vents connected with the molten reservoirs of the interior. Thus we conclude that they belong to an entirely different type, and have been built up in a different manner, from those represented by Etna, Vesuvius, and most of the extinct volcanoes of Auvergne, the Eifel, and of other districts considered in these pages.

Let us now endeavour to picture to ourselves the stages through which the moon may be supposed to have passed from the time her surface began to consolidate owing to the radiation of her heat into space; for there is every probability that some of the craters now visible on her disk were formed at a very early period of her physical history.

When the surface began to consolidate, it must also have contracted; and the interior molten matter, pressed out by the contracting crust, must have been over and over again extruded through fissures produced over the solidified surface, until the solid crust extended over the whole lunar surface, and became of considerable thickness.

It is from this epoch that, in all probability, we should date the commencement of what may be termed "the volcanic history" of the moon. We must bear in mind that although the moon's surface had become solid, its temperature may have remained high for a very long period. But the continuous radiation of the surface-heat into space would produce continuous contraction, while the convection of the interior heat would tend to increase the thickness of the outer solid shell; and this, ever pressing with increasing force on the interior molten mass, would result in frequent ruptures of the shell, and the extrusion of molten lava rising from below. Hence we may suppose the fissure-eruptions of lava were of frequent occurrence for a lengthened period during the early stage of consolidation of the lunar crust; but afterwards these may be supposed to have given place to eruptions through pipes or vents, resulting in the formation of the circular craters which form such striking and characteristic objects in the physical aspect of our satellite.[14]

It is not to be supposed that the various physical features on the lunar surface have all originated in the same way. The great ranges of mountains previously described may have originated by a process of piling up of immense masses of molten lava extruded from the interior through vents or fissures; while the great hollows (or "seas") are probably due to the falling inwards of large spaces owing to the escape of the interior lava.

But it is with the circular craters that we are most concerned. Judging from analogy with the lava-craters present on our globe, we must suppose them to be due to the extrusion, and piling up, of lava through central pipes, followed in some cases by the subsidence of the floor of the crater. It seems not improbable that it was in this way the greater number of the circular craters lying around Tycho, and dotting so large a space round the margin of the moon, were constructed. (See Fig. 38.) In general they appear to consist of an elevated rim, enclosing a depressed plain, out of which a central cone arises. The rim may be supposed to have been piled up by successive discharges of lava from a central orifice; and after the subsidence of the paroxysm the lava still in a molten condition may have sunk down, forming a seething lake within the vast circular rampart, as in the case of the Hawaiian volcanoes. The terraces observable within the craters in some instances have probably been left by subsequent eruptions which have not attained to the level of preceding ones; and where a central crater-cone is seen to rise within the caldron, we may suppose this to have been built up by a later series of eruptions of lava through the original pipe after the consolidation of the interior sea of lava. The mamelons of the Isle of Bourbon,[15] and some of the lava-cones of Hawaii, appear to offer examples on our earth's surface of these peculiar forms.

Such are the views of the origin of the physical features of our satellite which their form and inferred constitution appear to suggest. They are not offered with any intention of dogmatising on a subject which is admittedly obscure, and regarding which we have by no means all the necessary data for coming to a clear conclusion. All that can be affirmed is, that there is a great deal to be said in support of them, and that they are to some extent in harmony with phenomena within range of observation on the surface of our earth.

The far greater effects of lunar vulcanicity, as compared with those of our globe, may be accounted for to some extent by the consideration that the force of gravity on the surface of the moon is only one-sixth of that on the surface of the earth. Hence the eruptive forces of the interior of our satellite have had less resistance to overcome than in the case of our planet; and the erupted materials have been shot forth to greater distances, and piled up in greater magnitude, than with us. We have also to recollect that the abrading action of water has been absent from the moon; so that, while accumulations of matter had been proceeding throughout a prolonged period over its surface, there was no counteracting agency of denudation at work to modify or lessen the effects of the ruptive forces.

CHAPTER III.

ARE WE LIVING IN AN EPOCH OF SPECIAL VOLCANIC ACTIVITY?

At first sight we might be disposed to give to the question an affirmative reply when we remember the eruptions of the last few years, and add to these the volcanic outbursts and earthquake shocks which history records. The cases of the earthquake and eruption in Japan of November, 1891, where in one province alone two thousand people lost their lives and many thousand houses were levelled[1]; that of Krakatoa, in 1883; of Vesuvius, in 1872; and many others of recent date which might be named, added to those which history records;—the recollection of such cases might lead us to conclude that our epoch is one in which the subterranean volcanic forces had broken out with extraordinary energy over the earth's surface. Still, when we come to examine into the cases of recorded eruptions—especially those of great violence—we find that they are limited to very special districts; and even if we extend our retrospect into the later centuries of our era, we shall find that the exceptionally great eruptions have been confined to certain permanently volcanic regions, such as the chain of the Andes, that of the Aleutian, Kurile, Japanese, and Philippine and Sunda Islands, lying for the most part along the remarkable volcanic girdle of the world to which I have referred in a previous page. Add to these the cases of Iceland and the volcanic islands of the Pacific, and we have almost the whole of the very active volcanoes of the world.

Then for the purposes of our inquiry we have to ascertain how these active vents of eruption compare, as regards the magnitude of their operations, with those of the pre-historic and later Tertiary times. But before entering into this question it maybe observed, in the first place, that a large number of the vents of eruption, even along the chain of the earth's volcanic girdle, are dormant or extinct. This observation applies to many of the great cones and domes of the Andes, including Chimborazo and other colossal mountains in Ecuador, Columbia, Chili, Peru, and Mexico. The region between the eastern Rocky Mountains and the western coast of North America was, as we have seen, one over which volcanic eruptions took place on a vast scale in later Tertiary times; but one in which only the after-effects of volcanic action are at present in operation. We have also seen that the chain of volcanoes of Japan and of the Kurile Islands are only active to a slight extent as compared with former times, and the same observation applies to those of New Zealand. Out of 130 volcanoes in the Japanese islands, only 48 are now believed to be active.

Again, if we turn to other districts we have been considering, we find that in the Indian Peninsula, in Arabia, in Syria and the Holy Land, in Persia, in Abyssinia and Asia Minor—regions where volcanic operations were exhibited on a grand scale throughout the Tertiary period, and in some cases almost down into recent times—we are met by similar evidences either of decaying volcanic energy, or of an energy which, as far as surface phenomena are concerned, is a thing of the past. Lastly, turning our attention to the European area, notwithstanding the still active condition of Etna, Vesuvius, and a few adjoining islands, we see in all directions throughout Southern Italy evidences of volcanic operations of a past time,—such as extinct crater-cones, lakes occupying the craters of former volcanoes, and extensive deposits of tuff or streams of lava—all concurring in giving evidence of a period now past, when vulcanicity was widespread over regions where its presence is now never felt except when some earthquake shock, like that of the Riviera, brings home to our minds the fact that the motive force is still beneath our feet, though under restrained conditions as compared with a former period.

Similar conclusions are applicable with even greater force to other parts of the European area. The region of the Lower Rhine and Moselle, of Hungary and the Carpathians, of Central France, of the North of Ireland and the Inner Hebrides, all afford evidence of volcanic operations at a former period on an extensive scale; and the contrast between the present physically silent and peaceful condition of these regions, as regards any outward manifestations of sub-terrestrial forces, compared with those which were formerly prevalent, cannot fail to impress our minds irresistibly with the idea that volcanic energy has well-nigh exhausted itself over these tracts of the earth's surface.

From this general survey of the present condition of the earth's surface, as regards the volcanic operations going on over it, and a comparison with those of a preceding period, we are driven to the conclusion that, however violent and often disastrous are the volcanic and seismic phenomena of the present day, they are restricted to comparatively narrow limits; and that even within these limits the volcanic forces are less powerful than they were in pre-historic times.

The middle part of the Tertiary period appears, in fact, to have been one of extraordinary volcanic activity, whether we regard the wide area over which this activity manifested itself, or the results as shown by the great amount of the erupted materials. Many of the still active volcanic chains, or groups, probably had their first beginnings at the period referred to; but in the majority of cases the eruptive forces have become dormant or extinct. With the exception of the lavas of the Indian-Peninsular area, which appear, at least partially, to belong to the close of the Cretaceous epoch, the specially volcanic period may be considered to extend from the beginning of the Miocene down to the close of the Pliocene stage. During the Eocene stage, volcanic energy appears to have been to a great degree dormant; but plutonic energy was gathering strength for the great effort of the Miocene epoch, when the volcanic forces broke out with extraordinary violence over Europe, the British Isles, and other regions, and continued to develop throughout the succeeding Pliocene epoch, until the whole globe was surrounded by a girdle of fire.

The reply, therefore, to the question with which we set out is very plain; and is to the effect that the present epoch is one of comparatively low volcanic activity. The further question suggests itself, whether the volcanic phenomena of the middle Tertiary period bear any comparison with those of past geological times. This, though a question of great interest, is one which is far too large to be discussed here; and it is doubtful if we have materials available upon which to base a conclusion. But it may be stated with some confidence, in general terms, that the history of the earth appears to show that, throughout all geological time, our world has been the theatre of intermittent geological activity, periods of rest succeeding those of action; and if we are to draw a conclusion regarding the present and future, it would be that, owing to the lower rate of secular cooling of the crust, volcanic action ought to become less powerful as the world grows older.

APPENDIX.

A BRIEF ACCOUNT OF THE PRINCIPAL VARIETIES OF VOLCANIC ROCKS.

When leucite or nepheline replaces plagioclase, the rock becomes a leucite-basalt,[1] or nepheline-basalt. Some basalts have a glass paste, or "ground-mass," in which the minerals are enclosed.

The lava of Vesuvius may be regarded as a variety of basalt in which leucite replaces plagioclase, although this latter mineral is also present. Zirkel calls it "Sanidin-leucitgestein," as both the macroscopic and microscopic structure reveal the presence of leucite, sanidine, plagioclase, nephiline, augite, mica, olivine, apatite, and magnetite.[2]

Dolerite does not differ essentially from basalt in composition or structure, but is a largely crystalline-granular variety, occurring more abundantly than basalt amongst the more ancient rocks, and the different minerals are distinctly visible to the naked eye.

A remarkable variety of this rock occurs at Slieve Gullion in Ireland, in which mica is so abundant as to constitute the rock a "micaceous dolerite."

2. Gabbro.—A rather wide group of volcanic rocks with variable composition. Essentially it is a crystalline-granular compound of plagioclase, generally Labradorite and diallage. Sometimes the pyroxenic mineral becomes hypersthene, giving rise to hypersthene-gabbro; or when hornblende is present, to hornblende-gabbro; when olivine, to olivine-gabbro. Magnetite is always present.

These rocks occur in the Carlingford district in Ireland, in the Lizard district of Cornwall, the Inner Hebrides (Mull, Skye, etc.) of Scotland, and in Saxony.

3. Diorite.—A crystalline-granular compound of plagioclase and hornblende with magnetite. When quartz is present it becomes (according to the usual British acceptation) a syenite; but this view is gradually giving place to the German definition of syenite, which is a compound of orthoclase and hornblende; and it may be better to denominate the variety as quartz-diorite. The diorites are abundant as sheets and dykes amongst the older palæozoic and metamorphic rocks, and are sometimes exceedingly rich in magnetite. Mica, epidote, and chlorite are also present as accessories.

The rock occurs in North Wales, Charnwood Forest, Wicklow, Galway, and Donegal, and the Highlands of Scotland. There can be little doubt that amongst the metamorphic rocks of Galway, Mayo, and Donegal the great beds of (often columnar) diorite were originally augitic lavas, which have since undergone transformation.

4. Diabase.—It is very doubtful if "Diabase" ought to be regarded as a distinct species of igneous rock, as it seems to be simply an altered variety of basalt or dolerite, in which chlorite, a secondary alteration-product, has been developed by the decomposition of the pyroxene or olivine of the original rock. It is a convenient name for use in the field when doubt occurs as to the real nature of an igneous rock. Melaphyre is a name given to the very dark varieties of altered augitic lavas, rich in magnetite and chlorite.

5. Porphyrite (or quartzless porphyry).—A basic variety of felstone-porphyry, consisting of a felspathic base with distinct crystals of felspar, with which there may be others of hornblende, mica, or augite. The colour is generally red or purple, and it weathers into red clay, in contrast to the highly acid or silicated felsites which weather into whitish sand.

6. Syenite.—As stated above, this name has been variously applied. Its derivation is from Syene (Assouan) in Egypt, and the granitic rocks of that district were called "syenites," under the supposition (now known to be erroneous) that they differ from ordinary granites in that they were supposed to be composed of quartz, felspar, and hornblende, instead of quartz, felspar, and mica. From this it arose that syenite was regarded as a variety of granite in which the mica is replaced by hornblende, and this has generally been the British view of the question. But the German definition is applied to an entirely different rock, belonging to the felstone family; and according to this classification syenite consists of a crystalline-granular compound of orthoclase and hornblende, in which quartz may or may not be present. From this it will be seen that, according to Zirkel, syenite is essentially distinct from diorite in the species of its felspar.[3] It seems desirable to adopt the German view; and as regards diorites containing quartz as an accessory, to apply to them the name of quartz-diorite, as stated above, the name syenite as used by British geologists having arisen from a misconception.

7. Mica-trap (Lampophyre).—A rock, allied to the felstone family, in which mica is an abundant and essential constituent, thus consisting of plagioclase and mica, with a little magnetite. Quartz may be an accessory. This rock occurs amongst the Lower Silurian strata of Ireland, Cumberland, and the South of Scotland; it is not volcanic in the ordinary acceptation of that term. The term lampophyre was introduced by Gümbel in describing the mica-traps of Fichtelgebirge.

8. Andesite.—This is a dark-coloured, compact or vesicular, semi-vitreous group of volcanic rocks, composed essentially of a glassy plagioclase felspar, and a ferro-magnesian constituent enclosed in a glassy base. According to the nature of the ferro-magnesian constituent, the group may be divided into hornblende-andesite, biotite-andesite, and augite-andesite. Quartz is sometimes present, and when this mineral becomes an essential it gives rise to a variety called quartz-andesite or dacite.

These rocks are the principal constituents of the lavas of the Andes, and the name was first applied to them by Leopold von Buch; but their representatives also occur in the British Isles, Germany, and elsewhere. Dacite is the lava of Krakatoa and some of the volcanoes of Japan.

9, 10. Trachyte and Domite, etc.—These names include very numerous varieties of highly silicated volcanic rock, and in their general form consist of a white felsitic paste with distinct crystals of sanidine, together with plagioclase, augite, biotite, hornblende, and accessories. When crystalline grains or blebs of quartz occur, we have a quartz-trachyte; when tridymite is abundant, as in the trachyte of Co. Antrim, we have "tridymite-trachyte."

The trachytes occupy a position between the pitchstone lavas on the one hand, and the andesites and granophyres on the other.

(b.) Domite is the name applied to the trachytic rocks of the Auvergne district and the Puy de Dôme particularly. They do not contain free quartz, though they are highly acid rocks, containing sometimes as much as 68 per cent. of silica.

(c.) Phonolite (Clinkstone) is a trachytic rock, composed essentially of sanidine, nepheline, and augite or hornblende. It is usually of a greenish colour, hard and compact, so as to ring under the hammer; hence the name. The Wolf Rock is composed of phonolite, and it occurs largely in Auvergne.

(d.) Rhyolites are closely connected with the quartz-trachytes, but present a marked fluidal, spherulitic, or perlitic structure. They consist of a trachytic ground-mass in which grains or crystals of quartz and sanidine, with other accessory minerals, are imbedded. They occur amongst the volcanic rocks of the British Isles, Hungary, and the Lipari Islands, from which the name Liparite has been derived.

(e.) Obsidian (Pitchstone).—This is a vitreous, highly acid rock, which has become a volcanic glass in consequence of rapid cooling, distinct minerals not having had time to form. It has a conchoidal fracture, various shades of colour from grey to black; and under the microscope is seen to contain crystallites or microliths, often beautifully arranged in stellate or feathery groups. Spherulitic structure is not infrequent; and occasionally a few crystals of sanidine, augite, or hornblende are to be seen imbedded in the glassy ground-mass. The rock occurs in dykes and veins in the Western Isles of Scotland, in Antrim, and on the borders of the Mourne Mountains, near Newry, in Ireland.

11. Granophyre.—This term, according to Geikie, embraces the greater portion of the acid volcanic rocks of the Inner Hebrides. They are closely allied to the quartz-porphyries, and vary in texture from a fine felsitic or crystalline-granular quartz-porphyry, in the ground-mass of which porphyritic turbid felspar and quartz may generally be detected, to a granitoid rock of medium grain, in which the component dull felspar and clear quartz can be readily distinguished by the naked eye. Throughout all the varieties of texture there is a strong tendency to the development of minute irregularly-shaped cavities, inside of which quartz or felspar has crystallised out—a feature characteristic of the granites of Arran and of the Mourne Mountains.

12. Granite.—A true granite consists of a crystalline-granular rock consisting of quartz, felspar (orthoclase), and mica; the quartz is the paste or ground-mass in which the felspar and mica crystals are enclosed. This is the essential distinction between a granite and a quartz-porphyry or a granophyre. Owing to the presence of highly-heated steam under pressure in the body of the mass when in a molten condition, the quartz has been the last of the minerals to crystallise out, and hence does not itself occur with the crystalline form.

True granite is not a volcanic rock, and its representatives amongst volcanic ejecta are to be found in the granophyres, quartz-porphyries, felsites, trachytes, and rhyolites so abundant in most volcanic countries, and to one or other of these the so-called granites of the Mourne Mountains, of Arran Island, and of Skye are to be referred. Granite is a rock which has been intruded in a molten condition amongst the deep-seated parts of the crust, and has consolidated under great pressure in presence of aqueous vapour and with extreme slowness, resulting in the formation of a rock which is largely crystalline-granular. Its presence at the surface is due to denudation of the masses by which it was originally overspread.