In some few cases the lines of weakness which have been worn into caves, pot-holes, ravines, and valleys, may have been produced, as M. Desnoyers believes, by subterranean movements of elevation and depression; but in all those which I have investigated the faults do not determine the direction of the caverns. The mountain limestone of Castleton, in Derbyshire, offers an example of caves intersecting faults without any definite relation being traceable between them. The ramifications of the Peak cavern traverse the Speedwell Mine nearly at right angles, and the water flowing through it has been traced, Mr. Pennington informs me, to a swallow-hole near Chapel-en-le-Frith, running across two, if not three faults, which are laid down in the geological map. As a general rule caverns are as little affected by disturbance of the rock as ravines and valleys which have been formed in the main irrespective of the lines of fault.
M. Desnoyers points out the close analogy between caverns and mineral veins, and infers that both are due to the same causes. This, undoubtedly, exists in that class of veins which are known to miners as “pipe” and “flat veins;” and there is clear proof, in the majority of cases, that the cavities in which the minerals occur have been formed by the action of running water, and have subsequently been more or less filled with their mineral contents; and these have been deposited on the sides of the cavity by the same “incretionary[35]” action, as that by which dripstone is now being formed in the present caves from the solution of carbonate of lime. Such veins present every conceivable form of irregularity, and frequently contain silt, sand, and gravel, which have been left behind by their streams, and their history is identical with that of the caverns.
It is not so, however, with the second class of veins, the “rake,” “right running,” and “cross courses,” as the miners term them, or those which occupy lines of fault. The fissures which contain the ore are proved very frequently, by their scratched and grooved sides, and polished surfaces or slicken-sides, to have been the result of subterranean movements by which the rock has been broken by mechanical force. They have been subsequently modified, in various ways, by the passage of water, and filled with minerals, in the same manner as the preceding class. With this exception they present no analogy to the caverns, with which they contrast strongly in their rectilinear direction, as well as in their purely mechanical origin.
The various Ages of Caves.
It is very probable that caves were formed in calcareous rocks from the time that they were raised to the level of the sea, since they abound in the Coral Islands. “Caverns,” writes Prof. Dana,[36] “are still more remarkable on the Island of Atiu, on which the coral-reef stands at about the same height above the sea as on Oahu. The Rev. John Williams states—that there are seven or eight of large extent on the Island of Tuto; one he entered by a descent of twenty feet, and wandered a mile in one only of its branches, without finding an end to ‘its interminable windings.’ He says—‘Innumerable openings presented themselves on all sides as we passed along, many of which appeared to be equal in height, beauty, and extent to the one we were following. The roof, a stratum of coral-rock fifteen feet thick, was supported by massy and superb stalactitic columns, besides being thickly hung with stalactites from an inch to many feet in length. Some of these pendants were just ready to unite themselves to the floor, or to a stalagmitic column rising from it. Many chambers were passed through whose fret-work ceilings and columns of stalactites sparkled brilliantly, amid the darkness, with the reflected light of our torches. The effect was produced not so much by single objects, or groups of them, as by the amplitude, the depth, and the complications of this subterranean world.’”
Calcareous rocks might, therefore, be expected to contain fissures and caves of various ages. In the Mendip Hills they have been proved by Mr. Charles Moore to contain fossils of Rhætic age, the characteristic dog-fishes, Acrodus minimus, and Hybodus reticulatus, the elegant sculptured Ganoid fish, Gryrolepis tenuistriatus, and the tiny marsupials, Microlestes and its allies. This singular association of terrestrial with marine creatures is due to the fact, that while that area was being slowly depressed beneath the Rhætic and Liassic seas, the remains were mingled together on the coast-line, and washed into the crevices and holes in the rock.
The older caves and fissures have very generally been blocked up by accumulations of calc-spar or other minerals, and they are arranged on a plan altogether independent of the existing systems of drainage.
It is a singular fact that no fissures or caves should, with the above exception, contain the remains of animals of a date before the Pleistocene age. There can be but little doubt that they were used as places of shelter in all ages, and they must have entombed the remains of the animals that fell into them, or were swept into them by the streams. Caves there must have been long before, and the Eocene Palæotheres, and Anoplotheres met their death in the open pit-falls, just as the sheep and cattle do at the present time. The Hyænodon of the Meiocene had, probably, the same cave-haunting tastes as his descendant, the living Hyæna, and the marsupials of the Mesozoic age might be expected to be preserved in caves, like the fossil marsupials of Australia. The chances of preservation of the remains when once cemented into a fine breccia, or sealed down with a crystalline covering of stalagmite, are very nearly the same as those under which the Pleistocene animals have been handed down to us. The only reasonable explanation of the non-discovery of such remains seems to be, that the ancient suites of caves and fissures containing them, and for the most part near the then surface of the rock, have been completely swept away by denudation, while the present caverns were either then not excavated or inaccessible.
Such an hypothesis will explain the fact that the no ossiferous caverns are older than the Pleistocene age, not merely in Europe, but in North and South America, Australia, and New Zealand. The effect of denudation in rendering the geological record imperfect, may be gathered from the estimate, which Mr. Prestwich has formed, of the amount of rock removed from the crests of the Mendips and the Ardennes, which is in the one case a thickness “of two miles and more,” and in the other as much as “three or four miles.”[37] Under these conditions we could not expect to find a series of bone caves reaching far back into the remote geological past, since the caves and their contents would inevitably be destroyed.