2-14 DISINTBGBATION Chap. V. 



tljey are formed. Through these several 

 means, minute fragments of rocks of many 

 kinds and mere particles in the soil will be 

 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 



• This statement is taken from Mr. Julien, ' Proo. American 

 Assoc. Science,' vol. xxviii., 18Y9. d. 330. 



