CHAPTER XII.

Trap-pebbles of the boulder—Thickness of the earth's crust unknown—Not of much consequence to the practical geologist—Interior of the earth in a highly heated condition—Proofs of this—Granite and hypogene rocks—Trap-rocks; their identity with lavas and ashes—Scenery of a trappean country—Subdivisions of the trap-rocks—Intrusive traps—Trap-dykes-intrusive sheets—Salisbury Crags—Traps of the neighbourhood of Edinburgh—Amorphous masses—Contemporaneous trap-rocks of two kinds—Contemporaneous melted rocks—Tests for their age and origin—Examples from neighbourhood of Edinburgh—Tufas or volcanic ashes—Their structure and origin—Example of contemporaneous trap-rocks—Mode of interpreting them—Volcanoes of Carboniferous times—Conclusion.

Inthe previous pages, allusion has been made to the trap-pebbles imbedded in the boulder, to the various forms of decay exhibited by granitic and trappean rocks, and to the elevation and depression of the solid crust of the earth. Will the reader bear with me for but a few pages more, while I seek to indicate one or two points of interest in a branch of geology that would abundantly reward a diligent observer? Since the days of Hutton, the investigation of what are calledigneousrocks has fallen somewhat into the background, and geologists have given themselves, perhaps too exclusively, to the study of organic remains, so that while the palæontology of the British islands has enjoyed an extensive exploration, but little has been done towards the elucidation of our igneous formations and their accompanying phenomena. Much remains to be accomplished, even in those districts usually regarded as in a manner thread-bare, and he must be but an indifferent observer who cannot add a few gleanings to the general stock of information upon this branch of British geology.

Many conjectures have been formed, and many theories propounded, as to the nature of the internal parts of our globe. Some have supposed that there is an outer solid film or crust, some ten or twenty miles thick, enveloping a vast ball of intensely heated matter; others have attempted to show that the interior must be nearly solid throughout, with, however, great lakes, or vesicles of gas and melted rock, somewhat after the fashion, we may suppose, of the oil-holes in a Gruyère cheese. But whether the heated material occupy the whole or only parts of the internal area, is not of much consequence to the practical geologist; he is content to believe that it exists, and in sufficient quantity, too, to produce the most momentous changes on the surface of the earth. We see the effects of this subterraneous agent in earthquakes and volcanoes, and the geologist can tell us of similar, as well as of other changes, effected by it during past ages. Granite hills, and mountainous districts of mica-slate and gneiss, bear evidence of what is termedmetamorphism—a change in the mineral structure of rocks, believed to have taken place through the agency of heat deep in the interior of the earth; for no analogous appearances have been detected in progress at the surface. Such rocks, known asmetamorphic, orhypogene, still form a difficult problem, not likely to be satisfactorily solved until the chemist shall have thoroughly investigated the subject; for it seems likely to be found, after all, that long-continued chemical action, without a very alarming degree of heat, may have produced even the most intense metamorphism. But dropping this part of the subject, in which so much yet remains to be discovered, let us look for a little at another branch of the geologist's evidence, where we meet with no such hampering hypotheses and doubtful conjectures, namely, thetrap-rocks.

Every one knows that basalt, lava, pumice, scoriæ, and ashes, are the various matters ejected from volcanoes. When these materials are found interstratified among the various geologicalformations, they are termedtrap-rocks,—a name derived from the Swedishtrappa, a stair, in allusion to the step-like or terraced appearance which they often present. They are of all ages, having been detected in the lower Silurians of Wales, and in the deposits of all subsequent periods up to the volcanic eruptions of the present day; thus evidencing, that from the remotest times there have been Ætnas and Vesuvii slumbering perhaps for ages, and then awakening to lay the surrounding districts in ruins. I have already said that the rocks from which the geologist has to compile his history, are mostly relics of the sea; hence most of the trap-rocks which he meets with in his explorations are the products of submarine eruptions. Far away down among the Silurian rocks, he can trace the floor of a primeval ocean thickly covered with stone-lilies, trilobites, and molluscs, and in following it out he marks how ashes and lapilli, ejected from some submarine orifice, settled down amid the organisms and well-nigh destroyed them, while at other times streams of molten matter were poured out along the sea-bottom, and hardened into masses of solid rock. He sometimes even encounters what seems the vent whence these eruptions proceeded, filled up now by a boss or plug of hardened trap, but he never can detect any trace of land. Some of these oceanic volcanoes may, like Graham's Island in the Mediterranean, have raised their tops above water, sending clouds of steam and cinders far and wide through the air, but the waves would eventually wear down the new-born land, and scatter its broken fragments along the floor of the sea. Among the carboniferous rocks of Scotland, however, we meet with a different condition of things. There, too, we can trace out submarine lava-streams, and mark how showers of ashes destroyed the delicate organisms of the deep; but we encounter, besides, undoubted traces of a land, not parched and ruinous as though the igneous forces had laid it waste for ever, but thickly clothed with vegetation of a more luxuriant type than that which clustersover Vesuvius and Calabria, or lies spread out across the "level plains of fruit-teeming Sicily."[74]We have looked at the plants and animals of the Carboniferous era; its rivers and deltas; its slow elevations and depressions of the ground. It may, perhaps, complete the picture of that ancient period, if we examine, though but briefly, its igneous eruptions, the more especially since these may be regarded as, to a considerable extent, typical of trap-rocks belonging to every age and every country.

[74]Της καλλικἁρπον Σικελἱας λενροὑς γὑας. Æsch.Prom. Vinct.369—a passage graphically descriptive of an ancient eruption of Ætna.

Unless when deeply buried beneath drift-sand and clay, trappean regions usually possess scenery of a marked kind. A green undulating country stretches out as far as the eye can reach, diversified here and there with bold abrupt crags and conical hills. The lower grounds show in the winter season their rich brown loam, that waxes green as the spring comes on, and ere summer's close spreads out its heavy crops of golden grain. The higher ridges are for the most part thickly wooded, yet the soil is often scanty, and, among the white stems of the beech, or the matted roots of the fir and the elm, we may not unfrequently see the rock protruding its lichen-crusted face, mottled with mosses and liverworts, while some sluggish runnel collects in stagnant pools, or trickles over the blocks with a thick green scum. Sometimes the hill has never been planted, but stands up now, as it has done for centuries; its western face craggy and precipitous, with bushes of sloe-thorn and furze, and stray saplings of mountain-ash clinging to the crevices, while its eastern slope sinks down into the rolling country around with a green lumpy surface, through which, at many a point, the grey time-stained rock may be seen. The whole district suggests to the fancy a billowy sea, and, as one casts his eye from some commanding hill-top athwart the wide expanse of hill and valley, sweeping away in endless undulations, he is apt to bethinkhim of some day far back in the past, when the verdant landscape around lay barren and desolate, while the solid earth rocked and heaved in vast ground-swells like a wide tempested ocean. Such is the aspect presented by some of the more trappean regions of Scotland. But the origin of this kind of scenery must be ascribed to the effects of denuding currents in scooping out the softer strata into clefts and valleys, and leaving the harder trap-rocks in prominent relief, rather than to any great inequality of surface produced by the eruption of igneous matter; for we shall find that the throwing out of sheets of lava and showers of volcanic ashes was often a very quiet process after all.

Trap-rocks generally may be variously classified according to the aspect under which we view them. Mineralogically they areaugitic, when the mineralaugiteenters largely into their composition;hornblendic, when theaugiteis replaced byhornblende; andfelspathic, wherefelsparforms the most marked constituent. The first class includes all the dark homogeneous compounds calledbasalts; the second, the hornblendicgreenstones, ordiorites; and the third, thefelstones,porphyries, andtufas. Geologically, they arebedswhen they are interstratified with the contiguous rocks; anddykesorveinswhen they penetrate them like walls, or in an irregular manner. The former class may be either of the same age with the rocks among which they lie, or of a later date, just as in a pile of books the centre one may either have been placed there originally with the rest, or thrust in long afterwards. The latter class must always be later than the rocks which they traverse, for it is plain the rocks must have been in existence before trap-dykes and veins could be shot through them. Hence geologists are accustomed to speak of contemporaneous and subsequent trap-rocks: the one list including all the tufas, and those melted rocks which can be shown to have been erupted during the time when the limestones, sandstones, or shales around themwere forming; the other embracing all the dykes and veins along with those beds of melted rock which have been intruded between the strata. These and other distinctions will be better understood from a few examples collected chiefly from the carboniferous district of central Scotland.

The trap-rocks seen there exhibit a wide range of structure, texture, colour, and general aspect. There are two pretty marked kinds—the augitic or hornblendic, and the felspathic; the former being usually of a more or less crystalline aspect; the latter, commonly dull, and often without any crystals.[75]In the augitic traps, the crystals are sometimes of large size and well-defined, so that the rock could hardly be distinguished at first sight from an ordinary grey granite, while at other times, and not unfrequently even in other portions of the same mass, the stone assumes a black appearance without distinct crystals. The former variety would be called agreenstone, the latter abasalt; the chief components in either case being felspar and hornblende, or felspar and augite, with a variable admixture of other minerals, the shade of colour varying from a pale blue or leek-green, through the different hues of grey, to a deep velvet black. There are other traps, however, consisting entirely, or nearly so, of felspar, whence they are known asfelstones. Such rocks enjoy a wide range of colour, some of them being pure white, others of a bluish grey or dingy brown; and they may be seen graduating from a pale yellow, or flesh-colour, to a brick-red or deep purple. When a trap displays distinct disseminated crystals, usually of felspar, it becomes aporphyry; when it shows rounded cavities, like those of furnace-slag, it is said to bevesicular; and when these globular or almond-shaped cavities are filled with carbonate of lime, chalcedony, or other minerals, the rock forms anamygdaloid. Such peculiarities ofstructure indicate to some extent the origin of the mass, and may be found in any kind of trap. Thus we have porphyritic greenstones, basalts, or felstones, and the same rocks may be likewise vesicular or amygdaloidal. Some of them, such as many greenstones, display on weathered surfaces that curious spheroidal structure already alluded to; others are built up into geometric columns.

[75]This distinction, though a sufficiently safe one in some localities, must not be held as by any means universal in its application, the felspathic traps being often as crystalline in aspect as the augitic, and the augitic, on the other hand, as dull as the felspathic.

Such peculiarities of composition and structure form the basis of a mineralogical classification of the igneous rocks, which is of use in working out the geology of a district. The most convenient subdivision for our present purpose, however, is that which proceeds upon the origin and mode of occurrence of the trap-rocks. Viewed thus, they resolve themselves into two great groups, theintrusiveandcontemporaneous, both of which contain greenstones, basalts, &c.,—the sole distinction between those of the one class and those of the other, being the relation of age and mode of occurrence which they bear to the surrounding rocks.

I. Theintrusivetraps occur in the form of walls and veins, sometimes in that of flat parallel beds, and often as huge amorphous masses, to which no definite name can be given. But whatever shape they may assume, they generally agree in presenting well-marked features, whereby their origin can be readily ascertained. The rocks through which they pass are more or less hardened, often contorted, and sometimes traversed by innumerable cracks and rents, into some of which the trap has penetrated in the form of veins.

A trap-dyke is a long wall of igneous matter, cutting more or less perpendicularly through the surrounding rocks. Sometimes these dykes attain a breadth of many yards, and may not unfrequently be traced for miles running in a nearly straight line over hill and valley, easily recognisable by a long smooth ridge, with the rock protruding here and there from below where the soil is thin. It is interesting to follow out one of these long ramparts from its beginning to its close, and markhow undeviatingly it cuts through the rocks. No matter what may be the nature of the stone encountered, hard conglomerate, friable shale, compact limestone, or jointed fissile sandstone, all are broken across, and the right line preserved throughout. Nay, I have seen a still more curious instance of this persistency, where the dyke ran for four miles through a set of mountain limestone and lower coal-measure strata, and several enormous sheets of greenstone and basalt. Even when passing through these traps the dyke remained perfectly distinct, its crystalline structure and external configuration presenting a well-marked contrast with those of the surrounding eminences. Of course it must have been formed after all the rocks through which it passed. The sandstones and shales must have settled down long previously on some estuary bed or sea-bottom; the corals and shells of the limestones, and the matted plants of the successive coal-seams must have lived and died, perhaps thousands or millions of years before, and their remains have hardened into stone, ere the continuity of the strata was broken across by the long deep wall of greenstone. Trap-dykes are accordingly appropriately termedintrusive. They have been intruded among and must always be later than the rocks in which they occur. In tracing out their character, more especially in a trappean district, such as that of Linlithgowshire, where they abound, we soon find other evidence of their intrusive nature. Where they pass through limestone, they sometimes convert it into a white saccharine marble; shales they bake into a sort of porcelain or burnt pottery; and sandstones become semi-fused into a hard homogeneous quartz-rock. Nor are the changes confined to the rocks traversed; the dykes themselves, along their sides, become fine grained and hardened; occasionally, too, the colour alters from the usual bluish or greenish-grey to black, or to a brick-red, or dull-brown, similar to that of the altered shale and sandstone, of which detached portions may be found adhering to the outer walls of the dyke, or even embeddedin its substance. The central portion of the dyke may thus be markedly crystalline, forming what we should call a greenstone, while the outside parts, where the trap comes in contact with the adjacent rocks, are fine grained and homogeneous, so as to become a true basalt. Sometimes, too, these exterior edges are highly vesicular and amygdaloidal, detached fragments closely resembling the slag of an iron-furnace, and occasionally the dyke presents a columnar arrangement, the ends of the hexagonal or polygonal columns abutting against the sandstone or other rock on either side, and losing themselves towards the centre in the general mass of the trap. Where the strata traversed are broken and jointed, the dykes which cut them through may be seen in some places throwing out lateral veins that accommodate themselves to all the irregularities of the fissures. These minor portions exhibit for the most part the same leading features with the parent mass, and the result of the whole is a general baking of the beds, with sometimes not a little contortion, and an amount of irregularity and disturbance, apparent at once to the most inexperienced observer. (SeeFig. 34.)

If the reader will verify these statements by actual exploration in the field, he will probably not be long in arriving at the following conclusions: trap-dykes must once have been in a melted state, as is shown by their vesicular cavities and divergent veins; this liquid condition must have been attended with the most intense heat, as may be gathered from the burnt and baked appearance of the contiguous rocks; they have, for the most part, especially where of large size, risen from below along previously-formed dislocations—a circumstance which may be inferred from their persistency in a straight line through beds of very different resisting power, for had the liquid matter forced a way for itself, it would have squirted between the beds along the lines of least resistance, and not directly and for miles across them; and hence, trap-dykes must be regarded not asthemselves the agents in dislocating and contorting a district, but merely as signs of the parent force at work below.

All the features of these trap-dykes here stated may be observed in the central district of Scotland, among rocks of Carboniferous age. But he who would study trap-dykes on the great scale without quitting Britain, should visit some of the more trappean islands of the Hebrides. He will there find them intersecting glen and hill-side, in an intricate network, standing up through the heather like ruined walls, and running often for considerable distances up bald cliff-line, and across precipitous ravine. In some localities, among such limestone districts as that of Strath, detached eminences may be seen with congregated dykes coursing their sides and summits, while the heathy interspaces are cumbered with grey and white protruding blocks of marble, that give to these green knolls the aspect of old time-wasted abbeys with their clustering tombstones. The magnificent sections laid open in these localities by the action of mountain streams, and by the waves of the Atlantic, leave the student of igneous rocks nothing to desire save a long lease of leisure.

Another form frequently assumed by the intrusive traps, is that of wide beds or sheets intercalated with greater or less regularity among stratified rocks (Fig. 34b). They may be regarded as horizontal dykes, the igneous matter, in place of cutting across the strata, having forced a way for itself between them. Viewed in this light they will be found exactly to correspond with ordinary dykes; the rocks on which they rest, and those which lie above them being both altered like those on either side of a dyke or vein. A well-known example of this form of trap is that of Salisbury Crags, where a bed of greenstone twenty to eighty feet thick is intercalated among sandstones, shales, and coarse limestones, belonging to the Lower Carboniferous series. Its under surface presents a remarkably even line, broken at intervals, however, where the truncated ends of sandstone beds protrudeup into the greenstone, or where the latter cuts into the sandstone below, occasionally enveloping detached fragments, and sending veins through them. Along the line of contact both rocks undergo a change. The greenstone becomes reddened, finer grained, and of a dull earthy aspect. The sandstones and shales are also red, and excessively hard, the former resembling a quartz rock, and the latter passing into a sort of flinty chert or chalcedony. The sandstones above the trap, where they can be examined, are also found to present the same hardened, baked appearance, the most intense metamorphism being observable in those parts which are completely surrounded by igneous matter. These points were noted many years ago during the famous controversy between the disciples of Hutton and Werner, the former viewing them as demonstrative evidence of the igneous origin of the trap-rock, the latter, on the other hand, professing to see nothing in the section of the Crags at all militating against the theory that the rocks had originated from deposition in water. Many a battle was fought in this locality, and not a few of the trap-dykes and hills possess to the geologist a classic interest, from having been the examples whence some of the best established geological opinions were first deduced. The contest between the Huttonians and Wernerians terminated long ago in the acknowledged victory of the former; Hutton's doctrines are now recognised all over the world. It is interesting, however, to walk over the scenes of the warfare, and mark the very rocks among which it raged, and from the peculiarities of which it took its rise. Basalts and greenstones, sandstones and shales, with all their crumplings and contortions, still stand up as memorials of powerful igneous action, and of physical changes in the primeval past; and they have become to the geologist memorials, too, of changes in the onward progress of his science, where, out of conflicts perhaps yet more tumultuous than those of ancient Nature, there emerged at last the clear demonstrable truth.

Fig. 34.—Intrusive Trap.

In the accompanying section (Fig. 34), the more marked characters of intrusive traps are exhibited. The main mass of igneous rock is the dyke (d), rising through a dislocation or fault, which has thrown down the beds on one side several feet below those on the other, as is shown by the interruption of the shale and ironstone beds (sh). The dyke gives off two ramifications, one of them cutting across the beds obliquely as a vein (v); the other passing along the planes of the shaly layers as a horizontal bed (b). The vein, it will be noticed, produces considerable alteration in its progress, carrying up and baking a portion of the shale (sh), and turning up the edges of the beds on both sides, which get cracked and hardened along the line of contact. The bed runs with some regularity for a short distance through the shales, which show marks of great alteration at their junction with the trap. Its under surface at one point is seen to have involved a portion of the shale which has become in consequence highly metamorphosed, while along the upper surface the bed has sent out a shortirregular vein that twists and otherwise alters the shales above. These circumstances would suffice to show that even though we did not find this bed in connexion with a mass of intrusive trap, it must, nevertheless, have been thrust among previously-formed strata, and could not have been contemporaneous, that is, poured out along the sea-bottom before the shales above it were deposited.

But one other form needs to be mentioned here as characteristic of the Carboniferous intrusive trap-rocks—that of great amorphous masses which cut through the strata irregularly. They have not the wall-like form of dykes, nor do they conform to the line of bedding of the rocks among which they occur. They are sometimes irregular lumps, lying above or among the strata, and probably connected with some vein or dyke below. In other localities they look like the upper ends of vast pillars which may descend into the very depths of Tartarus, as though a great hole had been blown through the crust of the earth, and a column of melted matter had risen to fill the cavity. Such masses are often calledbosses, and seem not unfrequently to have been the craters of eruption whence great sheets of lava and showers of ashes were ejected far and wide over the neighbourhood. They serve to connect the intrusive traps, whose age is always more or less uncertain, with the bedded traps properly so called, the geological date of which can usually be sufficiently ascertained.

II. The bedded or contemporaneous trap-rocks consist of two well-marked kinds. There are, 1st, the melted rocks, such as greenstones and basalts and 2d, the tufas and volcanic ashes.

Those of the first-named class differ in no respect from the traps already noticed, so far as regards mineralogical texture, general structure, and appearance. In hand specimens the intrusive and bedded greenstones and basalts cannot be distinguished, nor even when examined in the field and in masses extending over considerable areas is it always possible to say towhich division any particular hill or crag should be assigned. The reason of this resemblance is obvious. Where a trap has either cut through or insinuated itself among rocks of earlier date it is called intrusive, in relation to the rocks so traversed, and of course we cannot be sure to what geological period it should be referred, nor how long an interval may have elapsed between the time when these rocks were forming and the time when the trap was intruded among them. If, however, the igneous rock passed upward through these same strata and then spread out as a flat sheet along the sea-bottom, the part that came to the top would be termed contemporaneous with the deposits going on at the time. Hence it follows that all contemporaneous lava-form trap-rocks are at the same time intrusive as regards the strata passed through in their progress to the surface. If the sheet of melted matter that spread out below the water were in the course of ages worn completely away, along with the strata subsequently piled above it, so as to leave merely a neck or dyke filling up the cavity through which the lava rose, we should pronounce the remnant intrusive, and could form no certain conclusion as to its age or as to whether its site had ever been a crater actively at work in throwing out lava and ashes. The sole difference, therefore, between a contemporaneous and an intrusive greenstone is simply this: the former rose through a fissure until it reached the surface, and then rolled out as a flat parallel sheet; the latter may have been erupted from below at the same time, yet, owing to different circumstances, never reached the surface, but spread out among or cut through the strata underneath. And so, when we come to examine in quarries, ravines, and other exposures, the remains of two such eruptions, we soon ascertain the relative age of the former from that of the strata among which it occurs, but as to the date of the latter we are wholly at a loss, for it gives us no clue by which we can show whether it was erupted before or after the other. We can butcompare the mineralogical character of the intruded with that of the contemporaneous masses in the same district, and, from the resemblance which may be traced between them, draw at the best but a doubtful inference as to their relative dates.

The contemporaneous traps always assume a bedded form, the intrusive occasionally do so; and the question naturally arises here, what are the tests whereby a bed of trap may be known to be contemporaneous and not intrusive? The answer is happily a simple one. An intrusive mass is found to alter to a greater or less extent the rocks in contact with it; if it occur as a dyke, then the beds on either side have been cut through and probably otherwise affected; if it take the form of a bed or sheet, the strata lying above and below it will be found to be both altered, showing evidently that a heated mass has been interposed between them, and consequently that the igneous rock is of later date than any of the strata among which it occurs. In the case of a contemporaneous melted trap, however, the appearances presented are different; it always takes the form of a flat bed corresponding to all the inclinations and curvatures of the sandstones, shales, limestones, or other strata among which it lies. If examined carefully, it may be found not unfrequently baking and contorting the bed that forms its pavement, but producing no change whatever on that which composes its roof. It may be capped and underlaid by layers of shale, and in such a case we might not improbably find the shale below it highly baked, so as to resemble a sort of rude pottery, while the shale above would present no sign of such metamorphism, but on the contrary might display its delicate plants or shells down to the very surface of the trap, and were the latter concealed from view we should never suspect, from the aspect of this shale, that any igneous rock existed in the neighbourhood. The inference to be drawn from such appearances seems very obvious. Had the upper shale been inexistence when the greenstone or basalt was erupted, it would have suffered an alteration similar to that produced on the shale below; and the fact, plain and palpable, that it has undergone no such change, shows pretty clearly that it was deposited at the bottom of the water after the trap had cooled and consolidated, and that consequently the trap must be intermediate in age between the beds on which it rests and those which lie above it; in other words, that it is acontemporaneousrock. Hence, if we know the exact geological position and age of the shales, we know also those of the associated trap, and can thus ascertain that at a certain definite period in the past history of our planet a particular district was the scene of volcanic action.

Examples of such contemporaneous traps abound among the carboniferous rocks of central Scotland, especially in Fife and the Lothians (Fig. 35). I may refer again to the vicinity of Edinburgh as affording some excellent illustrations. The eastern part of Arthur's Seat displays a series of basalts and greenstones which can be proved to have been thrown out during the times of the Lower Carboniferous rocks, at a period long anterior to that of the Burdiehouse limestone. The Pentland Hills exhibit on a much greater scale vast sheets of felspathic traps, such as felstones and tufas, traceable in some cases for six or seven miles, which were erupted at a still earlier period.[76]The trap pebbles in our boulder consisted of light yellow and pink felstone, and were derived, I make no doubt, from these Pentland Hill beds, when what forms now the cone of Carnethy, rising well-nigh 1900 feet above the sea, existed as one of a scattered archipelago of islets, or as a sunken rock battered by the waves that scattered its shingle along the floor of what may have been either a shallow sea or a shoaling estuary, where eventually thesand and pebbles hardened into that bed of coarse grey sandstone whence our boulder was derived.

[76]The geology of Arthur's Seat and Pentland Hills was admirably worked out more than quarter of a century ago by Mr. M'Laren. His work (already referred to) is unfortunately now out of print.

The second class of contemporaneous trap-rocks are the tufas or volcanic ashes. They differ entirely in their aspect and origin from any of the rocks already described. Greenstones, basalts, felstones, and such like, were all melted rocks, thrust up from below as we see lava thrown out by a modern volcano, being styled contemporaneous when poured out along the sea-bottom or the land, and intrusive when they never reached the surface but cut through the strata below. The tufas, however, point to a totally different origin. They are of various shades of colour, according to their chemical composition. In East Lothian they assume a deep red hue; among the Pentland Hills they are often flesh-coloured, while in Linlithgowshire they range from a dull-brown to a pale leek-green, green being the prevailing tint. They always show a dull uncrystalline surface, irregularly roughened by included fragments of various rocks, such as trap, sandstone, shale, and many others. These fragments orlapillivary in size from less than a pin-head up to large bombs of several hundredweight, and from being generally abundant give to the tufas one of their best-marked characteristics. The smaller pieces are usually more or less angular, and throughout the carboniferous series of Linlithgowshire consist chiefly of a pale felspathic matter, lighter in shade and commonly harder in texture than the matrix or paste in which they lie. In some localities, where the included pieces are larger, they have a rounded form, and often show a honey-combed vesicular surface, like balls of hardened slag. Fragments of sandstone have not unfrequently a semi-fused appearance, and plates of shale sometimes look like the broken debris from a tile-work, although in many instances these fragments may be found showing no trace whatever of alteration, being undistinguishable from the neighbouring sandstones and shales from which they probably came. I have seen in some of the coarsertufas, or rather volcanic conglomerates, enormous masses of basalt and greenstone buried deep in the surrounding green or red felspathic paste, and showing on their more prominent edges the usual vesicular cavities. In such conglomerates there is usually no division into beds; the whole mass, indeed, forms a bed between lower and higher strata, but internally it shows for the most part no trace of stratification. In these confused assemblages one may occasionally light upon detached crystals of augite or other mineral scattered irregularly through the tufa. Their angles will be found often blunted, and the crystals themselves broken, appearances which have likewise been noticed among the ash of modern volcanoes. When the tufas are finer grained they usually exhibit a well-marked stratification, and can often be split up into laminæ like an ordinary fissile sandstone. Organic remains not unfrequently abound in such laminated beds, and vary in their character as widely as in any other stratified rock, being sometimes land-plants, sometimes sea-shells.

Such are some of the more obvious characters of the volcanic ashes or tufas, as developed among the carboniferous rocks of central Scotland. Their great varieties of composition and general aspect render them a somewhat difficult set of rocks to master, but when fairly and fully understood they soon prove themselves to be by far the most interesting section of the traps, for one needs seldom to hesitate a moment as to their origin or date, while their fossil contents impart to them an interest all their own. By comparing such rocks with the consolidated ash or fine dust andlapilliof a modern volcano, a remarkable resemblance of external characters is found to subsist; and this likeness holds sufficiently close, when pursued into details, to show that the ancient and the modern rocks have resulted from the same source, that, namely, of volcanic eruption. The ash of active burning mountains falls down their sides loosely and incoherently, every successive shower ofdust or scoriæ settling without much regularity on those that have gone before. The ash of the old carboniferous eruptions, however, was showered for the most part over the sea or across wide shoaling estuaries, at least it is only such portions of it as fell there that have come down to our day. Settling down among the mud and sand at the bottom, the volcanic matter accumulated in wide horizontal beds, every marked inequality being smoothed down by the currents until a series of regularly stratified layers came to be formed, entombing any organisms that might find their way to the bottom or be lying there at the time. The ash of terrestrial volcanoes has no marked stratification because thrown out in open air, while that of the carboniferous rocks of central Scotland is distinctly bedded from having been deposited under water.

Tufas and contemporaneous melted traps are very generally found together interstratified regularly with each other, and the inference to be drawn from their juxtaposition is of course simply this, that at one time liquid lava rolled along the bottom of the water, while at another showers of volcanic dust and cinders settled down in successive beds. This active play of the igneous forces took place at the mouths of estuaries or farther to sea; and it is accordingly sometimes not a little interesting to trace, amid the sediment that accumulated below the water during the pauses between the eruptions, well-preserved remains now of plants that had come drifting from the land, anon of slim spirifers, and producti that swarmed upon the hardened lava-streams, and amid the thickening volcanic mud that slowly sank to the sea-bottom. Such a sequence of events will be made plain from the following section, the materials of which are derived from different parts of the trappean region of Linlithgowshire. The undermost bed here shown (1) is one of marine limestone, abounding with encrinal joints, corals, spirifers, and other undoubtedly marine organisms. Above it comes a layer of tufa or volcanic ash (2) of a dull green aspect, the boundaryline between the two rocks lying as clear as if the quarryman had marked it off with his foot-rule. The upper part of the ash, however, does not show an equally clear line of demarcation with the stratum above. On the contrary, it gradually changes its character, becomes more calcareous as it goes up, with here and there a stone-lily joint or a stray productus, until these organisms increase so much in number that the rock insensibly passes into an ordinary limestone (3) like that below. Next succeeds a thin seam of ash (4) resting sharply on the limestone and overlaid by a bed of shale (5) containing the same marine organisms. Another stratum of ash (6) resembling those below follows the shale, and is surmounted by a close compact greenstone (7) that hardens the ash on which it rests, but produces no apparent alteration on the soft fissile shale (8) above it. Next is a fourth seam of volcanic ash (9) resembling those below it, but without any shells or crinoidal joints, the only fossils observable being a few carbonized stems apparently of calamites and lepidodendra. Above it comes a bed of white quartzy sandstone (10) with similar vegetable remains, and then a layer of white stiff fire-clay (11) with rootlets of stigmaria, above which lies a seam of coal (12). A thin layer of soft blue shale (13) here intervenes, somewhat baked along its upper portions by another bed of compact vesicular greenstone (14), which displays in places a well-marked columnar structure. It is surmounted by a highly characteristic ash (15) in which there occur numerous large bombs chiefly of trap of different kinds, some of them highly vesicular. Fragments of shale also occur, mingled here and there with black carbonized fragments of coal-measure plants, but without any of the shells and other marine organisms so abundant below. The topmost bed is a grey carbonaceous sandstone (16), underlying a thin covering of vegetable mould.

Fig. 35.—Contemporaneous Trap.

Such is the skeleton, as it were, of the section; the mere dry bones which remain to the geologist, and which he must study closely to be able to give them life again. The lowest bed visible, with its stone-lilies and molluscs, we readily recognise as marking an old ocean-bed, so that the little episode in the primeval records of our planet here presented to us opens, like the two great epics of antiquity, within sound of the wide-roaring sea. The seam of ash which follows shows, from the sharpness of its line of demarcation with the limestone, how the denizens of the sea-bottom were suddenly destroyed by a thick shower of volcanic dust that settled down over their remains. The waters, however, soon cleared, and ere long stone-lilies and producti were plentiful as ever, mingling their remains among the upper layers of the soft muddy ash, and giving rise therefrom to a sort of calcareous ash or ashy limestone, until in the course of time the volcanic matter became wholly covered over by a seam of ordinary limestone. The corals and stone-lilies were, however, anew destroyed by the deposition of volcanic dust that settled over them as a seam of ash, after which the water was again rendered turbid and muddy by the inroad of foreign matter, which, brought down by rivers or by the changing currents of the ocean, sank to the bottom and eventually consolidated into a seam of shale. Thereafter the volcanic forces began once more to eject a quantity of dust and scoriæ that fell into the water and spread along the bottom as a stratum of ash, and to pour out a current of lava which hardened into a great sheet visible now as the undermost greenstone of the section.The emission of the lava seems to have terminated the eruption, for the next stratum is one of shale like that below the ash, so that the muddy sediment, the deposition of which was interrupted for a while by the volcanic products, began afresh to settle down along the sea-bottom. This last condition of things seems to have continued for a considerable period, seeing that the shale bed is relatively thick, and from its fissile laminated structure indicates a slow and tranquil deposition. Another eruption of volcanic dust and ashes again interrupted the detrital deposits, and gave rise to another seam of tufa. This last subterranean movement seems to have considerably altered the general contour of the sea-bottom, and so elevated it, at least at one part, that a thick accumulation of sand, and subsequently of clay, filled it up to the level of the water or nearly so, giving rise to a dense growth of the stigmaria and other coal-measure plants whose roots are still seen imbedded in the fire-clay on which, as a soft muddy soil, they originally grew. It is probable, however, that, notwithstanding such elevations of the sea-bed, there was a general subsidence of the ground during the accumulation of these strata, for we see that the peaty morass, represented now by the coal-seam, ere long sank beneath the waters, with the inroads of which it was unable to keep pace, while there slowly silted over it a muddy sediment that hardened at length into what is now a seam of shale. But this order of things had been in existence for but a comparatively short period when the igneous forces broke out again, ejecting a stream, of molten lava that spread along the bottom of the shallow waters and hardened as before into a sheet of greenstone. This was followed by an abundant shower of dust and lapilli, along with numerous large masses of greenstone and basalt. These falling into the water accumulated on the upper surface of the lava-stream, then somewhat cooled, and formed in the end a stratum of ash of a rubbly conglomeritic aspect. That the sheet of greenstone really spread out along the sea-bottom before theejection of the ash, and was not intruded among the beds at a later period,—that, in short, it must be regarded as a contemporaneous and not as an intrusive rock, seems sufficiently shown by its great regularity and evenness, and by the unaltered condition of the fine soft felspathic matter which covers its upper surface. It was assuredly in a highly-heated condition when poured out, as may be gathered from the baked aspect of the mud over which it rolled; but it had cooled and solidified, at least along its upper surface, ere buried beneath the shower of ashes. The last bed exhibited in the section is a grey sandstone, with many carbonaceous streaks and traces of land-plants, showing a pause in the volcanic activity of the district, during which the streams from the land brought down sandy sediment, with an abundant admixture of macerated leaves, branches, and other drift-wood.

It thus appears that not only were the plains and hills of the Carboniferous era richly clothed with vegetation, and its waters crowded with animals, but that then, as now, subterranean forces were at work, sometimes elevating, sometimes depressing the area alike of the land and of the sea; while, not unfrequently, melted lava rose from below, rolling along the bottom of the waters, and showers of ashes were flung far and wide through the air, settling at last as a thickening sediment along the floor of the sea, or athwart the marshy swamps of the delta. Whether the interior of the land had burning cones among its pine-covered hills we know not yet. Such, however, probably existed; nay, there may have been among the higher peaks some "snowy pillar of heaven," like the Ætna of Pindar, raising its smoking summit among everlasting crags of ice in solitudes lifeless and bare.[77]

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