CALCAIRE SILICEUX. PARIS GYPSUM. 



227 



shells. The total thickness of the beds of calcaire grossier, near 

 Paris, is about ninety feet. 



No beds of limestone resembling the calcaire grossier of Paris, 

 are found in the tertiary strata of England. The calcaire grossier 

 in the departments of La Dordogne and La Gironde, and other parts 

 of France, presents a considerable difference from that in the Paris 

 basin. In Hungary, extensive strata of the calcaire grossier have 

 been described by M. Beudant; they are, in every respect, analo- 

 gous to the strata in the Paris basin, both in their mineral and zoolo- 

 gical characters. The lower beds also are intermixed with shelly 

 sand, and green particles, which bear a close resemblance to the 

 shelly depositions in the plain of Lombardy. M. Humboldt thinks 

 he discovered, in some parts of South America, a formation similar 

 to the calcaire grossier. 



Calcaire silicevx is composed of limestone, sometimes grey and 

 compact, and sometimes tender and white : it is penetrated by silex 

 in every direction, and in all its parts. According to the early opin- 

 ion of M. Brongniart, the calcaire siliceux occupies the place of the 

 calcaire grossier where the latter is wanting; otfiers regard it as an 

 upper formation above tlie middle gypsum. Some of the beds of 

 the calcaire silicevx furnish mill-stones, and contain river shells. In 

 this bed, the silicate of magnesia was discovered by M. Brongniart. 

 The siliceous infiltrations sometimes form plates of chalcedony, and 

 mammillated concretions of chalcedonic chert, coloured red, violet, 

 and brown. 



Gypseous Marl and Gypsum. — This remarkable formation occurs 

 in detached hills along the course of the rivers Marne and the Seine; 

 it is supposed to have extended originally as one continuous bed 

 from east to west, twenty five leagues in length and eight in breadth : 

 its greatest thickness is about two hundred feet. 



The gypsum formation consists of alternating beds of gypsum and 

 argillaceous and calcareous marl, which are regularly arranged, and 

 preserve the same order of succession wherever they have been ex- 

 amined. The gypsum forms three distinct masses. The lowest 

 consists of thin strata of gypsum, containing crystals of selenite, 

 which alternate with strata of solid calcareous marl, and with argil- 

 laceous shale. The middle is like the lowest mass, except that the 

 strata of gypsum are thicker, and the beds of marl are not so nu- 

 merous ; it is chiefly in this mass that fossil fish are found. The 

 uppermost mass is the most remarkable and important of all ; it is 

 in some parts more than seventy feet thick ; there are but few beds 

 of marl in it; the lower strata of gypsum in this mass have a co- 

 lumnar structure: the gypsum is pure, and finely granular; it has a 

 light yellowish brown colour, which might perhaps more properly be 

 called a dirty white. In this upper mass of gypsum, the skeletons 

 and scattered bones of birds and unknown quadrupeds are discov- 

 ered ; sometimes, they are found in the solid gypsum, and some- 



