474 ON THE CONNECTION OF GEOLOGY 



they keep up a constant application of this substance to 

 the rock, and in this manner contribute indirectly to its 

 decomposition. There are some cryptogamic plants also, 

 which consume certain portions of the rocks with which 

 they are in contact, corrode their surface, and destroy 

 the cohesion of its parts, effects which may chiefly be 

 seen in certain cryptogamic plants attached to calcareous 

 rocks. In this manner one sort of vegetation prepares a 

 place for another, and the most imperfect vegetables are 

 subservient to the growth of the more perfect. 



After premising thus much, we shall now proceed to 

 the examination of the principal rocks, in so far as re- 

 gards their connection with the formation of productive 

 soil, beginning with those which resist decomposition in 

 the highest degree, and ending with those which are 

 the most conducive to the formation of loose earth and 

 soil. 



In the first class, we place those rocks which experience 

 no chemical decomposition, in so far as regards their prin- 

 cipal mass, and whose cohesion of parts is so great that 

 mechanical powers can only open their natural fissures 

 to a greater extent, and thus break them down into frag- 

 .ments. Of this kind are vitreous lava, pure quartz, com- 

 pact quartz, flinty slate, and porphyry with a siliceous 

 basis. On mountains consisting of these rocks, scarcely 

 any productive soil is found, and frequently none at all. 

 They are usually characterized by sterile rocks and cliffs, 

 the bases of which are covered with innumerable rough 

 fragments of stones, retaining their sharp edges for a 

 great length of time, the heaps of which seldom produce 

 any thing else than mosses, which frequently cover the 

 interstices of fragments, occasionally a few grasses, and 



