.yi Inhere Ores art chiefly found. [Book VI. 



All beds of .gravel are fuppofed to have been de- 

 pofited either by rivers or by the action of the fea, 

 and the ftones that compofe them to have been rounded 

 by attrition. It is no inconfiderable proof in -favour 

 of fuch a conjefture, that fea-fhells, &c. are fo fre- 

 quently found with gravel. 



The argillaceous ftrata are only productive of iron 

 and coal. The ores of copper, lead, zinc, &c. are 

 confined entirely to the lime-ftone ftrata, a few in- 

 ftances excepted,. where they are found in {hale. 



The difpofition of the fuperficial ftrata, however, 

 differs in mountainous and champaign countries. f In 

 a well which was dug at Amfterdam, to the depth .of 

 two hundred and thirty feet, the following fubftances 

 were found in fucceflion * : feven feet of vegetable 

 earth, nine of turf, nine of foft clay, eight of fand, 

 four of earth, ten of clay, four of earth, ten of fand, 

 two of clay, four of white fand, one of foft earth, 

 fourteen of fand, eight of clay mixed with fand, four 

 of fea- fand mixed with fhells, then an hundred and 

 two feet of foft clay, and then thirty- one feet of 

 fand. 



c In a well dug at Marly, to the depth of an hun- 

 dred feet, M. Buffon gives us a ftill more exact enu- 

 meration of its layers of earth. Thirteen feet of a 

 reddiih gravel, two of gravel mingled with a vitrifiable 

 fand, three of mud or (lime, two of marie, four of 

 marly ftone, five of marie in duft mixed with vitrifiable 

 fand, fix of very fine vitrifiable fand, three of earthy 

 marie, three of hard marie, one of gravel, one of eglan- 

 tine, a ftone of the hardnefs arid grain of marble, one 

 of gravelly marie, one of fiony marie, one of a coarfer 

 kind of ftony marie, two of a coarfer kind ftill, one of 



* Varemus, as quoted by M, Buffbn, p. 358: 



vitrifiable 



