376 Analyses of Boohs. [May, 



"5. A thick bed of white calcareous marl subdivided into 

 many strata by marl beds, and by a bed of zoned chert (silex 

 corne zonaire), almost jaspic, without either shells or magnesite. 



" 6. A bed about two decimetres thick, composed of brown 

 chert (silex corne) in irregular nodules, but principally flattened. 

 These are the nodules that are enveloped and even penetrated 

 by the Parisian magnesite of an isabella roseate grey colour. 

 It is sometimes very pure, does not effervesce with acids, and is 

 absolutely infusible in the heat of a porcelain furnace. It is 

 sometimes slightly translucent. 



" 7. These cherts (silex) are placed on a bed of hard calcare- 

 ous marl in nearly round nodules, and containing cyclostoma 

 mumia. 



" 8. Beneath is a thick bed 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 suspect its position by a compa- 

 rison 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 organic remains found in it ; now this character, 

 which is so useful in establishing analogies between formations 

 far distant from each other, possesses all its value 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 

 determination, and who have seen the cyclostoma mumia and 

 Linmeus longiscatus cited, have immediately lecognized the 

 position of the formation containing the magnesite of Coulom- 

 miers. 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 1 have just men- 

 tioned, having as yet been only found in the middle fresh water 

 formation, in that situated between the two marine formations of 

 the Paris basin, we should refer the magnesite of Coulommiers 

 to that fresh water formation ; it forms part, as we have else- 

 where* shown, of that which we have named siliceous lime- 

 stone. The hard calcareous marls, and the silex that accompa- 

 nies the magnesite, remind us of the siliceous and calcareous 

 characters of this deposit, and complete all the analogies. 



" The magnesite having shown itself in a very distinct manner, 

 both as to its purity and quantity in the siliceous limestone of 

 Coulommiers, the rules of geology teach us that we should find 

 it elsewhere, by searching for it in this formation ; this has in 

 fact happened. Proceeding towards Paris, and at about two 



* Description Oeologiquc des Environs de Paris, 1 822, p. 38, and 203. 



