of the Environs of Paris. 55 



The sea seems to have returned a third time, producing 

 some species of bivalve and turbinated shells ; but very soon 

 after the same sea gives birth to oysters only. Lastly, the 

 productions of the lowermost second sea reappear, and we 

 find on the top of Montraartrc the same shells which were 

 found at Grignon, and in the bottom of the quarries at 

 Gentilly and Meudon. 



Art. Vr. Formation of siliceous Limestone, 



The formation of which we have spoken is parallel, 

 as it were, to that of the siliceous limestone. It is neither 

 situate above nor below, but on one side of it, and seems 

 to retain its place in the immense extent of soil which it 

 covers to the west and south-west of Paris. 



This soil is placed immediately above some plastic clays. 

 It is formed of distinct beds of limestone, sometimes soft 

 and white, and sometimes gray and compact, and very fine 

 grained, penetrated with silcx which seems to have filtered 

 in every direction. As it is often porous, this silcx, by fil- 

 tering into the cavities, has fringed their sides with tufted 

 stalactites {mamelovces) variously coloured, or with very 

 short crystals of quartz, almost without any prism, but 

 clear and lim.pid. This disposition is very remarknble at 

 Champigny. The compact limestone thus penetrated \vith 

 silcx yiefds, on being burnt, lime of a very good quality. 



But the distinguishing character of this singular for- 

 mation, which no person had remarked before we did, 

 although it covers a considerable extent of soil, is this, 

 namely, that it contains neither marine nor fiuviaiile 

 fossiis ; at least we have not been able to discover any in 

 the great number of places where we have examined it with 

 the most scrupulous attention. 



It is in this soil that we find the stones known by the 

 name of burrstones {meuliere.s) : these stones, the origin, 

 formation, and situation of which were unknown to most 

 mineralogists, seem to be the siliceous impression of the si- 

 liceous limestone. The silex, stripped of its calcareous parts 

 by an unknown cause, must have left, and dcscs in fact 

 leave, porous but hard masses, the cavities of wh.ich-sfill 

 contain argillaceous marie, which prescntno trace of strati- 

 fication. We have made true artificial burrstones by ihrow- 

 ing this siliceous limestone into nitric aci(.l. We shall ex- 

 plain in our full Memoir the various districts of soil which 

 are formed of this limestone. We shall finish its general 



D 4 history 



