ch. xvur.]- 
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 grossier. — 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 mouthsof 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. Deshayes, Appendix I,, p. 26. 
