Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/257

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247
Mr. Webster on the Strata lying over the Chalk.

If we consider that the flinty chalk is somewhat above the level of the sea at Woolwich and Gravesend; that it dips under the Isle of Sheppey and disappears; that at Margate it has been so elevated that a considerable part of the lower chalk is now seen, the whole of the upper or flinty chalk being gone; that at Dover it rises to a vast height; that on the north side of the Thames it appears at Purfleet opposite to Gravesend, but immediately disappears to rise no more on the coasts of Essex and Suffolk.—If we reflect on the rapid dip of the chalk at the Hog's back, between Guildford and Farnham, with the many hills and hummocks of chalk to be seen in the counties of Suffolk, Norfolk, &c. we may perceive evident proofs of the great irregularity of the ancient surface of this stratum, and a part of the elements which may enable us to trace the limits of the land and sea at that period.

In speaking of the formation of the gravel, the probable mode has been detailed, by which the mountains of chalk originally appearing above the sea may have been worn away, cliffs have been formed, and the flints broken and rounded into pebbles. Of the early existence of this process, we have seen proofs in the vertical bed of pebbles of Alum bay, and in those frequently found in the sand of the plastic and blue clay in many other places. And also in the lower beds of the calcaire grossier of the basin of Paris.

We have seen also that the extraordinary event of the elevation or subsidence of the chalk of the Isle of Wight and Dorsetshire must have taken place after the deposition of the great stratum of blue clay.

A change of this kind, of which we have no parallel in human record, it would be in vain to endeavour to account for; but it must have been an event of itself sufficient to produce great changes in this part of the globe, and must have been accompanied by the most