28S M. Brongniart o;2 </?<? 



8. Beneath is a thick bed G of white calcareous marl, 

 friable or only splintery, and containing neither chert (silex) 

 nor shells. 



The total thickness of the beds composing this hill is nine 

 metres [about 29 feet]. 



As this succession of beds and rocks is Isolated, as no 

 other formation is seen above it, and as we do not know 

 that on which it rests, we can at most snspect its position by 

 a comparison of these rocks with those that resemble them 

 in the Paris basin ; but this is a presumption difficult to 

 prove without the presence of the or£;anic remains found in 

 it; now this character, which is so useful in estal)lishing 

 analogies between formations far distant from each other, 

 possesses all its ralue when it is required to determine the 

 position of one formation with respect to the others in the 

 same basin : it may then be here employed with perfect 

 safety, and geologists who admit these rules of determina- 

 tion, and who have seen the cyclostoma mumia and Limneus 

 longiscatus cited, have immediately recognised the position 

 of the formation containing the magnesite of Coulommiers. 

 These shells are not marine, one of them is evidently a fresh 

 water shell, consequently the magnesite belongs to a fresh 

 water formation, and the two species of shells I have just 

 mentioned, having as yet been only found in the middle 

 fresh water formation, in that situated between the two ma- 

 rine formations of the Paris basin, we should refer the mag- 

 nesite of Coulommiees to that fresh water formation ; it forms 

 part, as we have elsewhere * shewn, of that which we have 

 named siliceous limestone. The hard calcareous marls, and 

 the silex that accompanies the magnesite, remind us of the 

 siliceous and calcareous characters of this deposite, and com- 

 plete all the analogies. 



The magnesite having shewn itself in a very distinct man- 

 ner, both as to its purity and quantity in the siliceous lime- 

 stone of Coulommiers, the rules of geology teach us that 

 we should find it elsewhere, by searching for it in this for- 



• Description geologique des enTirons de Paris, 1822, p. 38, and 203. 



