Ch. XVIIL] 



PARIS BASIN. 245 



These observations relieve us from the difficulty of seeking a 

 cause why vegetable matter, and certain species of fresh-water 

 shells and a particular kind of clay, was first introduced into 

 the basin, and why the same space was subsequently usurped 

 by the sea. A minute examination of the phenomena leads us 

 simply to infer, that a river charged with argillaceous sediment 

 entered a bay of the sea and drifted down, from time to time, 

 fresh- water shells and wood. 



Calcaire grassier. The calcaire grossier above alluded to, is 

 composed of a coarse limestone, often passing into sand, such as 

 may perhaps have been derived from the aqueous degradation 

 of a chalk country. It contains by far the greater number of 

 the fossil shells which characterize the Paris basin. No less 

 than 400 distinct species have been derived from a single loca- 

 lity near Grignon. They are imbedded in a calcareous sand, 

 chiefly formed of comminuted shells, in which, nevertheless, 

 individuals in a perfect state of preservation, both of marine, 

 terrestrial, and fresh- water species, are mingled together* 

 and were evidently transported from a distance. Some of the 

 marine shells may have lived on the spot, but the Cyclostoma 

 and Limnea must have been brought there by rivers and 

 currents, and the quantity of triturated shells implies consider- 

 able movement in the waters. 



Nothing is more remarkable in this assemblage of fossil 

 testacea than the astonishing proportion of species referrible to 

 the genus Cerithium *. There occur no less than 137 species 

 of this genus in the Paris basin, and almost all of them in the 

 calcaire grossier. Now the living testacea of this genus inhabit 

 the sea near the mouths of rivers, where the waters are brackish, 

 so that their abundance in the marine strata of the Paris basin is 

 in perfect harmony with the hypothesis before advanced, that a 

 river flowed into the gulf, and gave rise to the beds of clay and 

 lignite before mentioned. But there are ample data for infer- 

 ring that the gulf was supplied with fresh water by more than 

 one river, for while the calcaire grossier occupies the northern 

 * See the tables of M. Desliayes, Appendix I., p. 26. 



