Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/241

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Mr. Webster on the Strata lying over the Chalk.

only to be seen as casts. In the numerous portions of a thin calcareous bed of a highly crystalline structure which lie scattered on the shore at Cowes, (but which I have not been able to find in situ) these Gyrogonites are found mixed with cerithia, bivalves, and a species of serpula; all these shells being in a whitened state. This fossil was formerly placed by the French naturalists among the multilocular shells, but from a late number of the Journal de Physique, it appears that recent observations have shown it to be the petrified seed of a species of chara.

It is singular that the Calcaire d'eau douce of the basins of Paris and of the Isle of Wight, though found so abundantly in both countries and constantly used as a building material, should have so long escaped the observation of naturalists. At the latter place it has been employed for building from time immemorial, not only in that island but in many places on this side of the channel, as at Portsmouth, Southampton, Lymington, &c.

§ 6. Alluvium.

Under this title may be comprehended all those collections of various materials, which have been transported at some former period from different parts of the globe, and deposited on the surface.

The whole of it is evidently composed of the detritus or fragments of substances which have been originally formed into regular strata, but which have been torn up and confusedly mixed together by violent and extraordinary causes, or gradually accumulated by rivers or meteoric agents. It is therefore, as might be expected, extremely various, according to the nature of the strata from which it has been derived.

Considered in this point of view the study of it becomes particu-