Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/245

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235
Mr. Webster on the Strata lying over the Chalk.

deep water it may in some degree be produced by the action of the tides; but it is by the irresistible force of the billows and breakers that it proceeds with the utmost rapidity. Hence all geologists, in examining the history of the strata, have considered rounded pebbles as proofs of the existence of land elevated above the water.

At this remote period of the earth, when the outline of the coasts were, as now, deeply indented by gulphs and bays, but whose forms and situations had but little if any correspondence with the present, great changes must have taken place by the gradual action of the sea; and vast accumulations of pebbles of different materials, formed by attrition, would be thrown upon the shores.

In the ancient Parisian gulph this phenomenon is distinctly pointed out; nor are similar appearances wanting to demonstrate the action of the same causes in the Isle of Wight and London basins.

We have seen that in Alum bay there are layers of rounded pebbles in the vertical sand strata, which must have been formed prior to the subversion of the chalk, and before the deposition of the London clay. Similar rounded flints, and often of a whitened appearance, are found in the sand strata at the bottom of the blue clay in various parts of the London basin.

All these belong to the most ancient flint pebbles, formed by the same sea that deposited our blue clay and the calcaire grossier of France.

The thick bank of flint pebbles at Woolwich and Blackheath, separated from the chalk only by sand, appears like the section of an ancient beach, it being not far distant from the shores of the ancient marine gulph or bay which now forms the London basin. The fossil shells which we see in such numbers among these peb-

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