<4 THE DENUDANTS, OR CARVERS OF THE 



can never occur, except in places that are naturally 

 subjected to extreme drainage ; such as chalk hills, a 

 country over cavernous limestone, a porous sandstone, 

 or the like. Calcareous rocks are, in a great measure, 

 chemically formed, and are more susceptible than 

 other rocks ; consequently they more readily suc- 

 cumb to meteoric abrasion, the carbonate of lime 

 being carried off in solution, while only the insoluble 

 parts, such as flint, chert, and clayey matter remain. 

 Besides, during warm weather such lands will be 

 scorched and burned by the sun, thus forming dust 

 to be carried away by wind or rain. 



Engineers are well aware of the protecting power 

 of grass, and they grow it on the slopes of their 

 railway cuttings and embankments, by which means 

 all denudation is stopped in two or three years ; 

 after which the surface is added to by the growth 

 and decay of vegetation, and the work of earthworms 

 and other soil-producing animals. 



Meteoric abrasion can perform a vast amount of 

 work in tropical climates, as pointed out in South 

 America by Agassiz, and in Abyssinia by Blandford 

 in his sketch of its geology. It should, however, be 

 remembered that meteoric abrasion is most active 

 in such climates, and that these, in by-gone ages, 

 could only have existed on limited portions of the 

 surface of the earth : yet, if we look back into the 

 records of the past, we find among the Laurentian, 



