470 ON THE CONNECTION OF GEOLOGY 



the disintegration of rocks, must depend upon the na- 

 ture of these rocks; its quality being determined by 

 the constituent parts of the rock from which it origina- 

 ted, and its quantity being proportioned to the greater 

 or less degree in which the rock may resist decomposi- 

 tion. 



The disintegration of rocks, and their conversion into 

 loose earth, are partly mechanical, and partly chemical. 

 The principal mechanical powers, by which disintegra- 

 tion is effected, are, \st, The weight of the loosened 

 parts ; 2*2, Water, not merely in its liquid and mobile 

 state, but also, and that chiefly, in the state of ice ; 3d, 

 The roots of vegetables in general, and especially of trees. 

 These powers usually act more or less in conjunction, 

 and the effects produced by this union are in many 

 cases almost incredible. 



The disintegration of rocks commences in those parts 

 where the power of cohesion is least energetic. Rents 

 take place owing to the unequal attraction of parts, and 

 also in the direction of planes, in which heterogeneous 

 parts are in contact; and in this manner the original 

 structure of rocks determines the first steps of their dis- 

 integration. Water, which enters into the minute fissures 

 of rocks, by the power of capillary attraction, is ex- 

 panded by congelation, and thus overcomes the cohesion 

 of parts, and produces rents. The roots of trees acting 

 as wedges, produce the same effect in a wonderful de- 

 gree, a phenomenon which has been so well illustrated 

 by Annaeus Seneca, in his Natural Questions. " Let us 

 consider ," says he, " how great a power is exerted by the 

 most minute seeds, which, although at first small as they 

 can scarcely find a place in the crevices of rocks, yet 



