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 
