Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/250

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Mr. Webster on the Strata lying over the Chalk.

up this stratum, of reducing it to fragments, and of disintegrating it, by removing the matrix containing the siliceous matter. And 3d. the deposition, either partial or general, of a solution of siliceous earth capable of cementing the whole together into one mass.

This formation is no doubt of ancient date, and long anterior to that of the present surface of the earth; a circumstance which is rendered probable by our finding masses of this pudding-stone itself water-worn, indicating, that since its cementation it has been subjected to some of these revolutionary processes, of which we have seen so many other proofs.

That all the ancient pebbles were not cemented together in a similar manner, is shewn by those in Alum bay, which are in loose sand; and by those which are found in the beds of sand in the blue clay, both in the neighbourhood of London and at Portsmouth.

But that some of the gravel may have been at a former period in the state of pudding-stone, and may have been disintegrated, is a circumstance also highly probable. Yet there does not appear any necessity for supposing that all our gravel has been originally pudding-stone, any more than that all sand has been sand-stone.

Another species of pebble which is very common, is a striped flint, or rather a kind of hornstone. These appear never to have had crusts, but owe their rounded forms entirely to the action of water. They may be supposed to have been derived from a bed of such a substance, which has been reduced to fragments. Beds of a striped chert very analogous to this I have seen in the island of Portland, alternating with the oolite.

Turf, so abundant in the north of France, is much less frequently seen in this part of England. The practice of draining, and the