Mr, Webster on the Strata lying over the Chalk. 237 



The siliceous limestone, which contains also a burr-stone, covers 

 half the bottom of the Paris basin. In the middle and upper beds 

 of the calcaire grossier they have hornstone, and the nectic and 

 lenticular quartz. The singular substance, called silex menilites, is 

 well known to all mineralogists : it is found in the foliated 

 argillaceous marls of the gypsums. In these marls they have also 

 siliceous nodules, which are white, opake, flat, and mammillated, 

 and also beds of flint. Silicified trunks of trees are also described. 

 The meuliere without shells, above the upper marine formation, 

 passes often into the state of flint, sometimes white and opake, 

 and sometimes grey and translucent. 



In their upper freshwater formation, a siliceous part containing 

 the same shells as the calcareous is very abundant. The description 

 of the several varieties of flints in these beds is as follows. 



1. Flint, of a grey colour, translucent, fracture close and waxy, 

 and even horny. 



2. Flint, yellow, very translucent, very easy to break, fracture 

 conchoidal and smooth. 



3. A jasper flint of an opake whiteness like wax, fracture 

 waxy and scaly, possessing little frangibility. 



4. An opake jasper flint, a little cellular, and having all the 

 characters of a compact burr-stone. 



Such is the imperfection of language, that a correspondence 

 merely in the general description of two mineral substances, is not 

 suflEicient to enable us to ascertain their identity. It must be allowed 

 however, that most of the characters above enumerated will apply 

 very accurately to many of the flints which I have described as 

 composing the greatest part of some heaps of our gravel. I have 

 also found among these many water-worn fragments of a siliceous 

 stone that bears a considerable resemblance to the French meuliere 

 or burr-stone. 



