] 824.] Dc la Becke's Selection of Geological Memoirs. 377 



leagues from Coulommiers, we observe near Crecy the same 

 rock with the same mineralogical circumstances ; i. e. the lime- 

 stone so compact that it resembles 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 

 resemblances 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 alto- 

 gether similar to that of Coulommiers ; the same limestone, 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. A rmet had remarked the presence of magnesite 

 in the marls of Montmartre ; M. Bayen had observed, more 

 than thirty years since, and had shown me that the menilite con- 

 tained 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, or in small masses, in all the 

 siliceous limestone rocks of this same formation, such as those 

 of Champigny, Orleans, Septeuil, &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 

 mentioned it. 



" Geological Circumstances of the Magnesite of different Places, 

 compared with 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 which surround our basin ; now, 

 it is remarkable, that we find the magnesite with all the circum- 

 stances which accompany it in that part of the Paris basin 

 where it is most pure. 



" Magnesite has long since been observed at Salinelle, near 

 Sommieres, in the department of the Gard, between Alais and 

 Montpellier ; but its position has only been determined a few 

 years since, by the description M. Marcel de Serre has published 

 of it. 



" It is therefore solely to the remarkable analogy of this posi- 

 tion with that of Coulommiers that I wish to call the attention 

 of naturalist*. The magnesite of Salinelle is schistose like that 

 of Coulommiers ; it possesses the same colour, approaching grey 

 with a roseate tint, with the same tenacity; it absorbs water in 

 the same manner ; it is composed of the same ingredients, i. e. 

 20 parts of magnesia instead of 24, 51 of silex instead of 54, 

 and 22 of water instead of 20. It will be acknowledged that it 



