220 CALCAIRE GEOSSIER. [Ch. XVI 



sea, a marine limestone was formed, another deposit of fresliwater origin 

 was introduced to the southward, or at the head of the bay. It is sup- 

 posed that during the Eocene period, as now, the ocean was to the north, 

 and the continent, where the great hikes existed, to the south. From that 

 southern region we may suppose a body of freshwater to have descended, 

 charged with carbonate of hrae and siUca, the water being perhaps in 

 sufficient vohune to freshen the upper end of the bay. 



The gypsum, with its associated marl and limestone, is, as before stated, 

 in greatest force towards the centre of the basin, where the calcaire gros- 

 sier and calcaire siliceux are less fully developed. Hence M. Prevost 

 infers, that while those two principal deposits were gradually in progress, 

 the one towards the north, and the other towards the south, a river de- 

 scending from the east may have brought down' the gypseous and marly 

 sediment. 



Gr^s de Beaucliamjp or Sables moyens^ B. 3. — In some parts of the 

 Paris basin, sands and marls, called the Gres de Beauchamp, or Sables 

 moyens, divide the gypseous beds from the calcaire grossier proper. These 

 sands, in which a small nummulite [N. variolaria) is very abundant, con- 

 tain more than 300 species of marine shells, many of them pecuhar, but 

 others common to the next division. 



Calcaire grossier^ upper and middle, B. 4. — The upper division of this 

 group consists in great part of beds of compact, fragile limestone, with 

 some intercalated green marls. The shells in some parts are a mixture of 

 Cerithium, Cyclostoma, and Corhula ; in others Lhnneiis, Cerithiurriy 

 Paludina, &c. In the latter, the bones of reptiles and mammalia, Paleo- 

 therium and Lophiodon, have been found. The middle division, or cal- 

 caire grossier proper, consists of a coarse limestone, often passing into 

 sand. It contains the greater number of the fossil shells which character- 

 ize the Paris basin. No less than 400 distinct species have been pro- 

 cured from a single spot near Grignon, where they are imbedded in a 

 calcareous sand, chiefly formed of comminuted shells, in which, never- 

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

 terrestrial, and freshwater species, are mingled together. Some of 

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

 and Limneus must have been brought thither by rivers and currents, 

 and the quantity of triturated shells implies considerable movement in 

 the waters. 



Nothing is more striking in this assemblage of fossil testacea than the 

 great proportion of species referable to the genus Cerithium (see p. 30, 

 fig. 44). There occur no less than 13 Y species of this genus in the Paris 

 basin, and almost all of them in the calcaire grossier. Most of the living 

 Ceritkia inhabit the sea near the mouths of rivers, where the waters 

 are brackish ; so that their abundance in the marine strata now under 

 consideration is in harmony with the hypothesis, that the Paris basin 

 formed a gulf into which several rivers flowed, the sediment of some 

 of which gave rise to the beds of clay and lignite before mentioned ; 

 while a distinct freshwater limestone, called calcaire siliceux, already 



