Page images


THERE are appearances in connexion with the Lias of Eathie which seem well suited to puzzle the geologist, and which have, in fact, already puzzled geologists not a little. We find them traversed by intrusive dikes of what seems a greyish-coloured trap, extremely obstinate in yielding to the hammer, and which stand up among the softer shales like the walls of some ruined village. They are trap-dikes in every essential except one ;-they occur in every possible angle of disagreement with the line of the strata : in some places they enclose the shale in slim insulated strips, as a river encloses its islands : in others they traverse it with minute veins connected with the larger masses, in the way in which granite is so often seen traversing gneiss : in yet others the limestone in contact with them seems positively altered;

the blue nodule has at the line of junction its strip of crystalline white, and the shale assumes an indurated and venous character: the dikes are, in short, trapdikes in every essential except one; but the wanting essential is of importance enough to constitute the problem in the case ;—they are not composed of trap. Some of our mineralogists have been a good deal puzzled by finding crystals of sandstone as regular in their planes and angles as if formed of any of the earths, or salts, or metals, whose law it is to build themselves up into little erections correctly mathematical in every point and line; and they have read the mystery by supposing that these sandstone crystals are mere casts moulded in the cavities in which crystals had once existed. The puzzle of the Lias dikes is of an exactly similar kind: they are composed, not of an igneous rock, but of a hard calcareous sandstone, undistinguishable in hand-specimens from an indurated sandstone of the Lower Oolite, which may be found on the shore beneath Dunrobin, alternating with shale-beds of the period of the Oxford clay. I succeeded in finding in it, on one occasion, a shell in the same state of keeping in which shells are so often found in the resembling rocks of Sutherland, but the species unluckily could not be distinguished. A common microscope at once detects the mechanical character of the mass; and I have learned that Dr. Fleming, after reducing a portion of it, sent him as an igneous rock, to its original sand, simply by submerging it in acid, expressed some little fear lest the sender should not have been quite 'up to trap.'

The explanation of the phenomenon seems rather difficult. There are instances in which what had once been trap-dikes are found existing as mere empty fissures; and other instances in which empty fissures have been filled up by aqueous deposition from above. An instance of the one kind is adduced, as the reader may perhaps remember, in the Elements of Lyell, from M'Culloch's Western Islands; two contiguous dikes traversing sandstone in Skye are found existing to a considerable depth as mere hollow fissures. An instance of the other kind may be found, says M‘Culloch, in a trap rock in Mull, which is traversed by a dike that, among its other miscellaneous contents, encloses the trunk of a tree, converted into brown lignite. In cases of the first kind, the original dike, composed of a substance less suited to resist the action of the weather than the containing rock, has mouldered away, and left the vent from which it issued a mere hollow mould, in which the semblance of a dike might be cast, just as the decay and disappearance of the real crystal is supposed to have furnished a mould for the formation of the sandstone one. In cases of the second kind, we see the fictitious dike actually existing : it is the sandstone crystal moulded and consolidated, and, in short, ready for the museum. And we have but to suppose the conditions of the two classes of dikes united,—we have but to suppose that the hollow filled by the aqueous deposition had been previously filled by an igneous injection, -in order to account for all the phenomena of an igneous dike accompanying a merely aqueous one.

We can scarce account in this way, however, for the formation of the dikes at Eathie, seeing that the shale in which they are included is of so soft and decaying a character, that no igneous rock could of possibility be more so ; nor, even were the case otherwise, could the upper portion of the dikes have existed as open chasms during the period in which the process of decay would have been taking place in the depths below. They would have infallibly filled up with the fragments detached from the sides and edges.

Mr. Strickland, in a paper on the subject in the Transactions of the London Geological Society, states the problem very strongly. “The substance of these dikes is such,' he says, 'that it is impossible to refer them to a purely igneous origin ;' and yet, however much ‘it may resemble an aqueous product,' it is as impossible to doubt that the dikes themselves are genuine 'intrusive dikes penetrating the Lias shale in all directions.' He adds further, as his ultimate conclusion in the matter, that the 'sedimentary structure of the rock forbids us to refer it to igneous injection from below;' and that, 'notwithstanding the complete resemblance of these intrusive masses to ordinary plutonic dikes, we have no resource left but to refer them to aqueous deposition, filling up fissures which had been previously formed in the Lias.' There is a peculiar rock in the neighbourhood, which throws, I am of opinion, very considerable light on their origin. It is what may be termed a syenitic gneiss, abounding in minute crystals of hornblende, that impart to it a greenish hue ; and in one place we find it upheaved so directly among the Lias beds, that it breaks their continuity. It raised them so high on its back, that the denuding agencies laid the back bare by sweeping them away. Let us but imagine that this disturbing rock began to rise under the earlier impulsions of the elevating agencies, and during the deposition of some one of the later secondary formations, as the precursor of the granitic range,—that the superincumbent Lias, already existing in its present consolidated state, opened into yawning rents and fissures over it, as the earth opened in Calabria during the great earthquake,-and that the loose sand and calcareous matter which formed the sea-bottom at the time, borne downwards by the rushing water, suddenly filled up these rents, ere the yielding matrix had time to lose any of its steepness of side or sharpness of edge, which it could not have failed to have done had the process been a slow one. The sandstone dikes, apparently Oolitic, mark, it is probable, the first operations of those upheaving agencies to which we owe the elevation of the granitic wall, and which, ere they accomplished their work, may have been active during occasional intervals for a series of ages. I am not of opinion that the accompanying marks of alteration among the shales and limestones of the beds are sufficiently unequivocal to render imperative some more fiery theory.



We find among the earliest bivalves of the Silurian system the delicate Terebratula, with its punctured umbone; we follow it downwards through all the various formations, and see it appearing on each succeeding stage, specifically new, but generally old, until, quitting the rocks with their dead remains, we pass to the existing testacea of our seas, and find among them the ancient Terebratula still extant as a living shell. Contemporary as a genus with every extinct form of animal life, we find it contemporary with the last of created beings also, --contemporary with ourselves; and the Terebratula is but one existence of a class to which, though their generic antiquity may be rather less remote, nearly the same remark applies. The ostrea still exists,-its congener and contemporary the gryphæa has perished; the nautilus survives,-its congener and contemporary the ammonite is long since dead; the cuttle-fish abounds on our shores,-its congener and contemporary the belemnite is to be found in only our rocks. And thus the list runs on. We can scarce glance over a group of fossils, whatever its age, which we do not find divisible into two classes of types,—the types which still remain, and the types which have disappeared. But why the one set of forms should have been so repeatedly called into being, and why the other set should have been suffered to become obsolete, we cannot so much as surmise. Why, it may be asked, should the nautilus continue to exist, and yet the ammonite have ceased with the ocean that deposited the Chalk ? or why should we have cuttle-fish in such abundance, and yet no belemnites? or why should not the gryphæa have been reproduced in every succeeding period with the oyster ? In visiting some old family library, that has received no accessions to its catalogue for perhaps more than a century, one is interested in marking its more vivacious classes of works,-its Spectators, and Robinson Crusoes, and Shakespeares, and Pilgrim's Progresses, in their first, or at least earlier editions, ranged side by side with obsolete, longforgotten volumes, their contemporaries, that died on their first appearance, and with whose unfamiliar titles one cannot connect a single association. But it is always easy to say why, in the race of editions, the one class should have been arrested at the very starting-post, and why the other should have gone down to be contemporary with every after production of authorship, until the cultivation of letters shall have ceased. It is otherwise, however, with the geologist. He finds he has exactly the same sort of fact to deal with,

« ՆախորդըՇարունակել »