Page:Natural History Review (1862).djvu/271

256 qu'ils convergaient des bords du bassin vers son centre, suivant les depressions préexistantes, et que leur élévation ou leur force de transport ne suffisait pas pour faire passer les debris qu'ils charriaient d'une de ces vallées dans l'autre."

Considering, however, all these facts, remembering that the constituents of the upper level gravels are, in all cases, derived from beds now in situ along the valley, that they have not only followed the lines of these valleys, but have done so in the direction of the present waterflow, and without in any case passing across from one river system to another, we may surely, I think, follow Mr. Prestwich in his conclusion that these gravels have been brought down, and deposited by the present rivers.

The sandstone blocks which occur in the gravel appear indeed at first sight to be irreconcileable with any such hypothesis. In some fits they occur frequently, and are of considerable size; the largest have myself seen is represented in the section, Fig. 1, taken close to the railway station at Joinville. It was 8 ft. 6 inches in length, with a width of 2 ft. 8 in., and a thickness of 3 ft. 4 in. Even when we remember that at the time of its deposition the valley was not excavated to its present depth, we must still feel that a body of water with power to move such masses as these must have been very different from any floods now occurring in those valleys, and might fairly perhaps deserve the name of a cataclysm. But whence could we obtain so great a quantity of water? We have already seen that the gravel of the Oise, though so close, is entirely different from that of the Somme, while that of the Seine again is quite different from that of any of the neighbouring rivers. These rivers therefore cannot have drained a larger area than at present; the river systems must have been the same as now. Nor would the supposition after all account for the phenomena. We should but fall from Scylla into Charybdis. Around the blocks we see no evidence of violent action; in the section at Joinville, the grey subangular gravel passed under the large block abovementioned, with scarcely any alteration. But a flood which could bring down so great a mass would certainly have swept away the comparatively light and moveable gravel below. We cannot therefore account for the phenomena by aqueous action, because a flood which would deposit the sandstone blocks would remove the underlying gravel, and a flood which would deposit the gravel would not move the blocks. The Deus ex machinâ has not only been called in most unnecessarily, but when examined turns out to be but an idol after all.

Driven, then, to seek some other explanation of the difficulty, Mr. Prestwich falls back on that of floating ice. Here we have an agency which would satisfactorily explain all the difficulties of the case. The "packing" and propelling action of ice would also account for some irregularities in the arrangement of the beds which