Chap. V. AND DENUDATION. 247 



continually exposed to chemical decomposi- 

 tion ; and thus the amount of soil will tend 

 to increase. 



As worms line their burrows with their 

 castings, and as the burrows penetrate to a 

 depth of 5 or 6, or even more feet, some 

 small amount of the humus-acids will be 

 carried far down, and will there act on the 

 underlying rocks and fragments of rock. 

 Thus the thickness of the soil, if none be re- 

 moved from the surface, will steadily though 

 slowly tend to increase ; but the accumulation 

 will after a time delay the disintegration of 

 the underlying rocks and of the more deeply 

 seated particles. For the humus-acids which 

 are generated chiefly in the upper layer of 

 vegetable mould, are extremely unstable com- 

 pounds, and are liable to decomposition before 

 they reach any considerable depth.* A thick 

 bed of overlying soil will also check the 

 downward extension of great fluctuations of 

 temperature, and in cold countries will check 

 the powerful action of frost. The free access 

 of air will likewise be excluded. From these 



* This statement is taken from Mr. Julien, ' Proc. American 

 Assoc. Science,' vol. xxviii., 1879, p. 330. 



