Page:Popular Science Monthly Volume 2.djvu/367

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to the conditions under which the implement-hearing drifts are found; for if the term petrological is to be understood as meaning rocks found in situ in the river-basins, and thus native to the soil, then it is not the fact that the constituents of the gravels in question belong to those basins; for we know that they are often largely made up—in one instance cited by Mr. Evans, to the extent of 50 per cent.—of the quartzose stones known as Lickey pebbles, and rounded fragments of jasper, quartz, and other foreign rocks. Such rocks certainly do not belong petrologically, in the proper sense of that term, to the river-basins in which they occur, but to strata of a far earlier date. As Dr. Buckland has shown, the quartzite pebbles are derived from the New Red sandstone beds in Warwickshire and Leicestershire, and were at some remote period forced over the escarpment of the Oolite into the south and east of England. Whether they were brought in before or after the present river-valleys were formed is not very clear, nor perhaps very material. It is incontestable that they were transported from a great distance, and possibly by the same forces that brought the flint-gravels; and it is equally certain, in several instances, that their transport cannot be attributed to rivers now in action, because those rivers flow, as at Brandon, toward the quarter from which the stones were brought.

Nor, if it were certain that the intrusion of these rocks dated back to the Glacial epoch, as is usually supposed, or to some other very distant period, and had thus become denizens, if not natives of the soil, could the inference which is drawn from the absence of extraneous rocks be regarded as satisfactory.

The occurrence of alternate elevations and depressions of the land above or below the sea-level, during the post-glacial times, has been suggested by several English writers; and, if we suppose a district comprising the south of England and the north of France, corresponding, or nearly so, with that in which no bowlder-clay is found, to be sufficiently depressed, and then invaded by a deluge, the argument drawn from petrological conditions will cease to apply; for, no rocks are found in the drift-gravels, but such as belong to the supposed deluge-basin. A delude of short duration would not necessarily introduce any foreign rocks into the submerged area, but would sweep into hollows and valleys those that came in its way; and, even should the submergence be of long continuance, as in some provinces of Holland, it would leave no more traces than those exhibited in our drift-gravels. That such a partial deluge was both possible and probable is evident, when it is considered that a depression of 600 feet would perfectly well effect it; and, as we have evidence that the land has risen in several places 30 feet and more within the historical period, it is not difficult to believe that, in the infinitely longer time that probably intervened after the Glacial epoch, the same process of elevation may have been going on for many ages.