Magiiesite of the Paris Basin, S^c. 2Sf 



thation ; this has in fact happened. Proceeding towards 

 Paris, and at about two leagues from Couloinmiers, we ob- 

 serre near Crecy the same rock with the same mineralogical 

 circumstances ; i. e. the limestone so compact that it resem- 

 bles the fine compact limestone of the Jura, the chert (silex), 

 the clayey marls, the magnesite, but less pure, and the same 

 fresh water shells. 



The short distance of these two places rendered these re- 

 semblances very presumable ; but transporting ourselves to 

 St. Ouen, close to Paris, on the bank of the Seine and at the 

 foot of Montmartre, we find the magnesite in a formation 

 altogether similar to that of Coulommiers ; the same lime- 

 stone, the same chert (silex), the same shells occur there ; 

 the position of the rock beneath the gypsum is there well 

 determined. The magnesite is however less pure here and 

 less apparent ; traces of it only occur ; these traces had long 

 since been observed. M. Armet had remarked the presence 

 of magnesite in the marls of Montmartre ; M. Bayen had 

 observed, more than thirty years since, and had shewn me 

 that the menilite contained it. Now this belongs to the 

 fresh water formation beneath the gypsum ; it is probable 

 that we should find this mineral either in minute quantities, 

 oriu small masses, in all the siliceous limestone rocks of this 

 same formation, such as those of Champigny, Orleans, Sep- 

 teuil, &c. I have recognised it in a greyish clayey marl 

 which accompanies a silex resinite of the environs of Mans, 

 consequently at more than 40 leagues to the west of Paris, 

 and 50 leagues from the first place in which I have men- 

 tioned it. 



Geolegical circumstances of the magnesite of different placesy 

 compared zeith those of the Parisian magnesite. 



We shall find this rock still further distant, in a basin se- 

 parated from ours not only by a distance of more than 120 

 leagues, but by chains of mountains whose structure and 

 nature are altogether foreign to those Mhich surround our 



