Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/588

456 account of convenience of access or better materials. And instead of being blended with, the higher beds (with some exceptions, which are probably attributable to the displacement and reconstruction of the original deposit), they are found in the lowermost stratum—that which rests immediately upon the Cretaceous or Tertiary beds, and which would be left bare when these became dry land.

It is true that occasionally, although comparatively rarely, implements are found lying above the lowest bed; but this is not inconsistent with the belief that they were originally deposited at the lower level, which I regard as their ordinary or normal position. Unlike marine deposits, which are usually those of accretion, the changes which are effected by river- or deluge-agency are those of denudation and dislocation; and it is evident that after the lowest stratum was constituted it was overwhelmed by other very considerable masses, which could only have been transported by powerful torrents. These, in their course, conveying rocks of considerable magnitude, could not fail to some extent to break up the subjacent beds, and in this way some implements would be taken from their first place of deposit, and left at relatively higher levels. As every inundation disturbs, and partially, if not entirely, effaces the traces of that which preceded it, so the upper portion of any bed may be carried away and form a new deposit, upon which the lower portion may afterwards be thrown, and thus the position of each becomes reversed. The worn appearance of some specimens, as compared with the fresh and sharp condition of others, tends to show that, while the latter have been but little, if at all, displaced, the former have been subjected to much rolling and attrition.

The circumstance that some implements are found lying above the lowermost stratum of gravel is thus not inconsistent with the belief that the stones from which they were wrought were nevertheless taken from that bed, and that they are thus to be ascribed to the commencement of the Quaternary period. On the other hand, if we should reject this view, we should be forced to conclude that the process of manufacture was continued through the very lengthened periods that would be requisite for the excavation of the existing river-valleys, involving the assumption, so difficult of acceptance, that although these implements were fabricated and used by many successive generations, they all passed away without leaving any other trace of their existence; and we must also suppose a constant recurrence of those agencies, whatever they may have been, by means of which these things were buried under thick masses of gravel and sand.

Under all the circumstances, therefore, it seems probable that the implements for the most part were not transported by any river or flood, but that they were made, or left, at or near the spots where we find them, although in some instances, especially in valleys and watercourses, they may have been afterwards displaced.

But even if it were otherwise, if it were certain that they were brought into their present position at the same time with the beds of gravel in which they occur, and by the same means, the belief that