Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/592

460 The presence of freshwater shells in some of these beds has been regarded as indicative of their fluviatile origin; as far, however, as the evidence at present extends, this condition is the exception rather than the rule. When these gravels remain at what I regard as their original place of deposit, viz. immediately upon the chalk or gault surface, I believe that not a single river or lake shell has ever been found associated with them, or lying below them, either in the flint-implement-bearing gravels, or in the very extensive beds of similar gravel in the south and south-east of England and in the north of France, which are not known to contain implements. Certainly no shells have been seen in either of the four deposits above described; nor are there any at the Reculvers, Bournemouth, or Hill Head; and the gravels at Madras are equally destitute of them. The presence of these shells in the upper portion of these beds by no means involves the conclusion that they are of the same age as the underlying drift; for, assuming that that was formed and left as above suggested, the valleys and hollows would soon become lakes and rivers, into which in process of time freshwater mollusks would find their way; and whenever the lower beds should be broken up and reconstructed, the shells would be mingled with the débris, and thus become undistinguishable from the older deposits.

Another circumstance which has been relied upon as showing the fluviatile origin of these gravels is the absence of any rocks except those of the district through which the rivers take their course. I do not think that this is very clearly established ; but, assuming it to be so, it is quite consistent with the notion of diluvial transport that the loose objects found on the surface should not be transported out of their own district: rocks and stones, if swept away by a deluge, would very soon find their way into valleys and hollows, and be left there when the waters had retired.

In conclusion I would suggest that the distribution, in the first instance at least, of these drift-beds, containing, as they do, so large an admixture of chalk and tertiary and boulder-clay rocks, may reasonably be attributed to the same forces or conditions, whatever they were, by which the Tertiaries and Boulder-clays were broken up and their materials so widely dispersed and intermingled. We know nothing of these, except from their results; but, whatever they may have been, it seems quite certain that they are not ascribable to fluviatile agency, and I am therefore disposed, with the French geologists, to attribute them to some powerful cataclysmal action, perhaps of short duration, and several times repeated.