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 short irregular 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 called bosses, 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 to which 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 but compare 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 in existence 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 a contemporaneous rock. 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 the sand 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 or lapilli vary 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 coarser tufas, 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 and lapilli of 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 of dust 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.