278 MINERALOGY OF THE ENVIRONS OP PARIS. 



either above or below it. It rests immediately on the 

 plastic clay. It consists of strata, not only of a white 

 limestone, but also of a gray, compact, or fine granular 

 limestone, which is penetrated in all directions with sili- 

 ca ; and its numerous cavities are lined with siliceous 

 stalactites, or quartz crystals. It is destitute of petrifac- 

 tions. A species of millstone sometimes occurs in it, which 

 appears to be the siliceous limestone deprived of its cal- 

 carious ingredient by some agent unknown to us. This 

 rock is scarcely entitled to the rank of a distinct forma- 

 tion : it appears to be one of the members of the pre- 

 ceding series without petrifactions. It may be remarked 

 that it is not uncommon to observe in the same forma- 

 tion beds with and without petrifactions. 



FIFTH AND SIXTH FORMATIONS. 



Fresh Water and Marine Origin- 

 Gypsum Formation, and the Marine Marl Formation* 

 This formation is not entirely of gypsum, but contains 

 also beds of clay marl and calcarious marl. These are ar- 

 ranged in a determinate order when they all occur together, 

 which, however, is not always the case. They lie over the 

 coarse marine limestone ; and the gypsum, which is the 

 principal mass of the formation, does not occur in wide 

 extended plateaus, like the limestone, but in single conical 

 or longish masses, which are sometimes of considerable 

 extent, but always sharply bounded. Montmartre presents 

 the best example of the whole members of the formation, 

 and there three beds of gypsum are to be observed su- 

 perimposed on each other. 



,, . .. ,, - .3 ,i.*\ r < '' ;j ; "^5* / '7 - 



The first bed consists of alternate layers of gypsum 

 solid caicarious marl, and of thin slaty argillaceous marl 

 or adhesive slate. The layers of gypsum are thin, and 



