HUTTONIAN THEORY. 107 



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thing but the conftant and flow difintegration of 

 the rocks, f n the permanence, therefore, of a coat 

 of vegetable mould on the furface of the earth, 

 we have a demonflrative proof of the continual 

 deftrudion of the rocks ; and cannot but ad- 

 mire the fkill, with which the powers of the 

 many chemical and mechanical agents employ- 

 ed in this complicated work, are fo adjufted, 

 as to make the fupply and the wafte of the foil 

 exadly equal to one another. 



104. Before we take leave of the rivers and 



the plains, we mull remark another fad, often 

 " obferved in the natural hiftory of the latter, and 



clearly evincing the former exiftence of imraenfe 

 bodies of ftrata, in lituations from which they 

 have now entirely difappeared. The fadl here 

 it' alluded to is, the great quantity of round and 



hard gravel, often to be met with in the foil, 

 under fuch circumftances, as prove, that it can 

 only have come from the decompofition of rocks, 

 that once occupied the very ground over which 

 this gravel is now fpread. In the chalk coun- 

 try, for inftance, about London, the quantity of 

 flints in the foil is every where great ; and, in 

 particular fituations, nothing but flinty gravel 

 is found to a confiderable depth. Now, the 

 fource from which thefe flints are derived is 

 quite evident, for they are prccjfely the fame 

 with thofe contained in the chalk beds, v/here- 



ever 



