Trap-rocks generally may be variously classified according to the aspect under which we view them. Mineralogically they are augitic, when the mineral augite enters largely into their composition; hornblendic, when the augite is replaced by hornblende; and felspathic, where felspar forms the most marked constituent. The first class includes all the dark homogeneous compounds called basalts; the second, the hornblendic greenstones, or diorites; and the third, the felstones, porphyries, and tufas. Geologically, they are beds when they are interstratified with the contiguous rocks; and dykes or veins when 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 them were 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 a greenstone, the latter a basalt; 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 as felstones. 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 a porphyry; when it shows rounded cavities, like those of furnace-slag, it is said to be vesicular; and when these globular or almond-shaped cavities are filled with carbonate of lime, chalcedony, or other minerals, the rock forms an amygdaloid. Such peculiarities of structure 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, the intrusive and contemporaneous, 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. The intrusive traps 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 mark how 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 termed intrusive. 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 embedded in 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. (See [Fig. 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 as themselves 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. 34 b]). 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 protrude up 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.
