Mr. Webster on the Strata lying over the Chalk, 231 



only to be seen as casts. In the numerous portions of a thin calca- 

 reous 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 mul- 

 tilocular 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 Ports- 

 mouth, 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 m.ight 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- 



