STUDY IV. 169 



nos ; the difference of their fubftances fuppofes 

 different caufes in their formation. 



Agates, flints, and every fpecies of the filex, 

 feem to be analogous to vitrifications, from their 

 half-tranfparency, and from their being ufually 

 found in beds of marl, which refemble banks of 

 lime extinguifhed ; but thefe fubftances are not 

 the productions of fire, for lavas never prefent any 

 thing fimllar. I have picked up, on the flinty 

 hills of lower Normandy, oyfter-fhells perfeftly 

 complete, amalgamated with black flints, which 

 they call bifets. Had thefe bifets been vitrified 

 by fire, they would have calcined, or, at leaft, al- 

 tered the oyfter-fhells which adhered to them ; 

 but thefe were as found as if juft taken out of the 

 water. The fhelving fea-coaft along the diftriâ: 

 of Canx, are formed of alternate ftrata of marl 

 and bifets, fo that, as they are cut perpendicularly, 

 you would call it a great wall, of which the layers 

 had been regulated by an Architeâ: ; and with (o 

 much the greater appearance of probability, that 

 the people of the country build their houfes of the 

 fame materials, difpofed in the felf-fame order. 



Thefe banks of marl are from one to two teet 

 broad, and the rows of flints which feparate them, 

 are three or four inches thick. I have reckoned 

 feventy or eighty of fuch horizontal ftrata from 



the 



