PLASTIC CLAY FORMATION. 



also in the ge.nus, from those that occur in the coarse lime- 

 stone- Two species of belemnite occur in the chalk, and 

 these appear to be different from those found in the lime- 

 stone, and are considered to characterize it. 



The chalk forms the bottom of the basin or gul in 

 which are deposited tjie different formations that occur 

 around Paris. Its surface must have presented numerous 

 inequalities before the present strata were deposited over 

 it, because we observe promontories and islands of chalk 

 rising through the newer formations. 



SECOND FORMATION. 



Fresh water Origin.* 



Plastic Clay. 



All around Paris, we find the chalk covered with a de- 

 posite of plastic clay, which is dug and used in the manu- 

 facture of different kinds of pottery. This clay varies in 

 colour, being white, gray, yellow, red, and black, some- 

 times contains a layer of sand, very rarely (only the 

 purer varieties) organic remains, viz. cytherea, turtellae, 

 bituminous wood, and in some places fragments of chalk 

 have been observed in it. It is neither intermixed with 

 the chalk at its line of junction with it, nor is it more 

 calcarious where in contact with that mineral, than 

 at a distance from it ; hence Cuvier conjectures, that it 

 has been deposited after the chalk, and is therefore a se- 

 parate formation. 



* I designate the formationsyresTi water and marine, according to the 

 idea of Cuvier and Brongniart ; although I do not agree with these 

 philosophers in their opinion of the alternate play of salt and fresh 

 water. 



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