CHAPTER XIII

Summary

CRAG OF NORFOLK AND SUFFOLK.

The older Pliocene strata, described in the last chapter, are all situated in countries bordering the Mediterranean, but we shall now consider a group in our own island, which belongs to the same era. We have already alluded to this deposit under the provincial name of crag, and pointed out its superposition to the London clay, a tertiary formation of much higher antiquity. The crag is chiefly developed in the eastern parts of Norfolk and Suffolk, from whence it extends into Essex.

Its relative age—A collection of the shells of the ‘ crag’ beds, which I formed in 1829, together with a much larger number sent me by my friend, Mr. Mantell of Lewes, were carefully examined by M. Deshayes; and compared to the tertiary species in his cabinet. This comparison gave the following result: out of lll species, 66 were extinct or unknown, and 45 recent, the last, with one exception (Voluta Lamberti, Sow.), being now inhabitants of the German ocean. Such being the proportion of recent and extinct species, we may conclude, according to the rules before laid down that the crag belongs to the oldser Pliocene period.

Mineral composition—So heterogeneous is this deposit in mineral character, that we can scarcely convey any correct notions of its appearance, without describing the beds separately in the different localities where they occur. In general, they consist of sand, gravel, and blue or brown marl—the shells imbedded in the sand and marl being, for the most part, broken and sometimes finely comminuted.