SECOND FfcESH WATER FbR&ATiOtf. 289 



The flint is sometimes nearly pure; sometimes approaches 

 to pitchstone or to jasper and quartz; and, lastly, it 

 has a corroded shape when it has all the characters of true 

 millstone, but which is in general more compact than the 

 millstone without shells. The limestone of this formation 

 is white or yellowishlgray ; sometimes nearly friable, like 

 marl or chalk; sometimes compact and solid, with a fine 

 grain and cdnchoidal fracture : the concholdal varieties 

 are rather hard, but easily broken into sharp-edged frag- 

 ments, somewhat like flint. These characters apply only 

 to the limestone near Paris; Tor, at a considerable distance, 

 the limestone occurs very compact, of a grayish-brown co- 

 lour, and which readily cuts and polishes. The limestone 

 of Mont-Abusar, near Orleans, which contains bones of 

 the Palaeotherium, belongs to this formation* Even the 

 hardest varieties of this limestone^ after exposure to the 

 air for a time, soften ; and hence it is used as a marl for 

 manuring the ground. All the varieties, both hard and 

 soft, are traversed by empty vermicular cavities, whose 

 walls are sometimes of a pale green colour. Where the 

 siliceous minerals and the limestone are intermixed, the 

 latter is always corroded, full of cavities, and its irregu- 

 lar cells are filled with calcarious marl. Tire essential 

 character of this formation is, that it contains fresh water 

 and land shells, nearly all of which belong to genera that 

 now live in our morasses, but no marine shells ; at least 

 in such places as are distant from the subjacent marine 

 formation. The following is a list of those fossil organic 

 remains that belong particularly to the upper fresh water 

 formation. 



Cyclostoma elegans antiquum, 

 Potamides Lamarkii. 

 Planorbis rotundatus. 

 cornu. 



37 



