

MINERALOGY OF THE ENVIRONS OF PARIS. 



TENTH FORMATION. 

 Fresh water Origin. 



TAe Flint and Siliceous Limestone Formation. 



We have already described a formation which, accord- 

 ing to Cuvier, has been deposited from fresh water, be- 

 cause the fossil animals it contains are analogous to those 

 we find in our fresh Crater lakes. This formation, which 

 consists of gypsum and marl, is separated from another 

 and more superficial fresh water formation, of which we 

 are now to give an account, by the upper marine sand- 

 stone already described. 



The second fresh water formation, in the vicinity 

 of Paris, consists of two sorts of stone,, flint and siliceous 

 limestone. These substances sometimes occur indepen- 

 dent of each other; in other instances they are intimately 

 mixed together. The nearly pure limestone is the most 

 common ; the next in frequency is a mixture of flint and 

 iimestone ; but large masses of pure flint are the rarest. 



of sand, extends nearly throughout the whole plateau, and varies in thick- 

 ness from three to five fathoms ; but millstones cannot be made of every 

 portion of the mass; hence we must not expect to find it throughout 

 the whole bed. A bed of rolled masses of millstone, about a foot and 

 half thick, lies over it ; over this a thin bed of iron-shot sand, contain- 

 ing smaller pieces of millstone, and above this bed is one of sand, from 

 12 to 17 yards thick. If the stone rings when struck with a hammer, 

 it will answer for large millstones. The bed never affords more than 

 three millstones in the direction of its thickness. It frequently hap- 

 pens, that the fissures allow the workmen to extract the masses in a 

 perpendicular direction, and these are the best. Millstones are formed 

 Ijy joining many of these parallelopipedal pieces together, and confin- 

 ing the whole with an iron hoop. These pieces are exported frous 

 Franco, to England and America. 



