Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/212

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Mr. Webster on the Strata lying over the Chalk.

and this is found only in the lower beds, a circumstance which they have pointed out as particularly indicative of them.

The lowest bed is also particularly distinguished by the vast number of fossil shells contained in it; the greatest part of which are more unlike the recent shells than those of the superior beds. The fossil shells of this bed are often well preserved, and are easily detached from their matrix; and many of them have still their original pearly lustre.

The middle beds also inclose a great many shells; one of them is called the green bank, from its colour. It often contains impressions of leaves and stalks of land vegetables, mixed with cerithia, ampullariæ, and other marine fossils.

The next bed of calcaire grossier has less variety of fossil shells than those just mentioned, but the cerithia are very abundant in it: and towards the top there is a thin bed, containing a prodigious quantity of corbulæ, long shaped, and striated. Above these are the marls, both calcareous and argillaceous; with calcareous sand, sometimes agglutinated, and which contains hornstone with horizontal zones.

The uppermost beds of this system have much fewer shells; and these are generally altogether wanting at the top.

In passing over in review all the strata which we know above the chalk in England, there does not appear to be any one that can be considered as exactly corresponding to this considerable and well marked stratum in the basin of Paris; and in instituting a comparison between the English and French strata, were we to require a perfect agreement in all the beds, we should here totally fail.

Yet although we cannot point out a calcareous rock precisely similar, and possessing the same importance of character, I think a sufficient number of circumstances may be shewn to justify our