Calcaire grossiere and Paris Gypsum. 171 



the proper characters assigned to such deposits by Brongniart. 

 When the marine limestone was depositing, the rivers were con- 

 veying into the basin mud, vegetable and animal matter, and 

 some fresh water shells ; the mud and sand were mixed with 

 the fragments of shells and various marine bodies, and have thus 

 formed the coarse marine limestone in the spots where we find 

 it ; while there were formed elsewhere, either alone or mixed up 

 with gypsum, strontianite and silex, in the locaUties where 

 these matters were forming, and where no marine animals were 

 living ; or they accumulated where the currents carried them, 

 and were precipitated nearly alone, when they did not advance 

 far into the basin. In this way was formed, in the inferior part 

 of the Parisian deposits, pot an alternation of marine and fresh 

 water deposits, but a marine deposit of many beds, where we 

 find fresh water shells, mixed or not mixed, in some localities, 

 with marine shells. The number of these can only be deter- 

 mined approximately ; it is a local accident ; we know already 

 two or three instances of the first kind, in the marine limestone, 

 one in the Gres de Beauchamp and d'Eranville, one in the up- 

 per gypsum ; and for the second case, we have the marly beds, 

 with paludines, below and above the Gres de Beauchamp, in the 

 high road of Maffiers, and below the gypsum, the shelly rocks 

 of St Ouen, the shelly marls of the gypsum, small beds with 

 paludines among the oyster beds of Montmartre, and the silice- 

 ous limestone above gypsum, and not below, as M. Brongniart 

 conceived, — a fact well known, since the fortifications were made 

 upon the heights near Nogent sur Marne and Fontaine aux 

 Bois. Perhaps even the Cytheria bed is a case of fresh water 

 fossil. According to our explanation, the singular mixture and 

 entangling of the gypsum and marine limestone, observed by 

 Prevost, is a natural thing ; fresh water shells were isolated in 

 some marine limestone, as, for instance, the cyrene at Passy ; 

 some others were entangled in marls or gypsum ; beds of clay, 

 with lignite, were formed in the marine limestone, and would as 

 the gypsum envelope particular vegetables and animals. Last- 

 ly, silica being distributed irregularly, by means of mineral wa- 

 ter, was precipitated in particular localities, and produced in one 

 part of the basin, more exposed to the afflux of river water, the 

 well known siliceous limestone, in which fresh water fossils are 



