Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/242

232 interesting, since it enables us to trace back, in some degree, the great changes which have taken place upon the surface of the earth.

In that part of our island which we are now considering, this alluvium or covering is of a nature peculiar to it. Besides the vegetable earth, clays, marls, and sands, which it possesses in common with other places, it is distinguished by a vast quantity of rounded siliceous pebbles of various kinds and sizes, which lie distributed in a very unequal manner, sometimes forming thick beds intermingled with clay, sand, and small sharp fragments of flints, at other places mixed with shells of various kinds, and sometimes almost without any other substance. This compound is termed Flint gravel.

When we observe a heap of these pebbles, we easily see that they consist of a great variety of kinds, and upon attentively examining them we are able to reduce this variety to several classes.

Some are evidently fragments of the flinty nodules originally belonging to the chalk strata. This is evinced by their mineralogical characters, their sharp conchoidal fracture, peculiar black colour, and by portions of the white crusts with which they were invested while in the chalk beds still remaining attached to them.

In others this origin is not so evident, the crusts having been entirely worn off, and the fragments themselves rounded by attrition. Yet their fracture, colour, and other circumstances, oblige us to suppose that these also were derived from the chalk. In many places the whole or the greater part of the gravel consists of these rounded chalk flints; and hence probably some have been induced to suppose that all the pebbles of the London gravel have proceeded from the same source.

But besides these other siliceous nodules occur, the origin of