Ch. XVIII.] PARIS BASIN. 247 



the gypsum, and the white and green marls, subdivisions 

 of No. 3 of the table of Cuvier and Brongniart. These 

 were once supposed to be entirely subsequent in origin to the 

 two groups already considered ; but M. Prevost has pointed 

 out that in some localities they alternate repeatedly with the 

 calcaire siliceux, and in others with some of the upper mem- 

 bers of the calcaire grossier. The gypsum, with its associated 

 marls and limestone, is in greatest force towards the centre 

 of the basin, where the two groups before mentioned are less 

 fully developed ; and 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 descending 

 from the east may have brought down the gypseous and marly 

 sediment. 



It must be admitted, as highly probable, that a bay or 

 narrow sea, 180 miles in length, would receive, at more points 

 than one, the waters of the adjoining continent ; at the same 

 time we must observe, that if the gypsum and associated green 

 and white marls of Montmartre were derived from an hydro- 

 graphical basin distinct from that of the southern chain of lakes 

 before adverted to, this basin must nevertheless have been 

 placed under circumstances extremely similar ; for the identity 

 of the rocks of Velay and Auvergne with the fresh-water group 

 of Montmartre, is such as can scarcely be appreciated by geo- 

 logists who have not carefully examined the structure of both 

 these countries. 



Some of our readers may think that the view above given of 

 the arrangement of four different sets of strata in the Paris 

 basin is far more obscure and complicated than that first pre- 

 sented to them in the system of MM. Cuvier and Brongniart. 

 We admit that the relations of the several sets of strata are less 

 simple than the first observers supposed, being much more ana- 

 logous to those exhibited by the lacustrine groups of Central 

 France before described. 



The simultaneous deposition of two or more groups of strata 

 in one basin, some of them fresh-water and others marine, must 



