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 G, 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. 



