40 NATURAL HISTORY. 



but those rocks are placed on other rocks ; tlicre air 

 mountains upon mountains, and rocks upon rocks, to 

 such a considerable height, and in so great an extent 

 of ground, that we can scarcely be certain where there 

 is earth at bottom, and of what nature it is. We set- 

 peaked rocks which are many hundred feet high ; these 

 rocks rest on others, which perhaps are no less so ; 

 nevertheless, may we not compare great with small ? 

 and since the rocks of little mountains, whose bases 

 are to be seen, rest on the earth less heavy and solid 

 than stone, may we not suppose that the base of hiyh 

 mountains is also of earth ? 



In a soil where flint is the predominant stone, the 

 country is generally fertile, and if the place is unculti- 

 vated,, and these stones have been long exposed to the 

 air, without being moved, the upper superfices is al- 

 ways very white, while the opposite side, whi:h touches 

 the earth} preserves its natural colour. If the black- 

 est, and most flinty flint be exposed to the weather, in 

 less than a year its surface will change colour ; and if 

 we have patience to pursue this experiment, we shall 

 see it by degrees lose its hardness, transparency, and 

 other specific characters, and approach every day near- 

 er and nearer the nature of argile. 



What happens to flint happens to sand ; each grain 

 of sand may be considered as a small flint, and each 

 flint as a mass of grains of sand, extremely fine and 

 exactly grained. The example of the first degree of 

 decomposition of sand is found in the brilliant and 

 opake powder sailed Mica, in which potters earth and 

 slate are always diffused. The entirely transparent 

 flints, the Quariz, produce, by decomposition, fat and 

 soft talc, as pctrih'able and ductile as clay : and it ap- 

 pears to me that talc is a mediate term between glass or 



