456 ON THE CONNECTION OF GEOLOGY 



tifically of agriculture and forestry have usually looked 

 upon the vegetable soil in its own simple capacity, with- 

 out regard to its foundation and origin. To point out 

 the way by which we are to proceed in our investigation 

 of the relations which exist between the solid crust of 

 the earth and the productive soil which covers it, is the 

 principal object of the following observations. 



Bare rocks cannot be made subservient to the purposes 

 of agriculture. Lichens indeed, cover the surface of 

 rocks, deriving their chief nutriment from the atmos- 

 phere ; mosses draw the water necessary for their sub- 

 sistence from the fissures of stones ; the roots of grasses 

 seek in the chinks of rocks for particles of earth sufficient 

 for their sustenance ; various shrubs and trees penetrate 

 here and there into rocky masses by their roots (having 

 the powerful and continued action of living wedges), 

 where the cohesion of the parts is smallest, in order to 

 prepare a fixed seat for themselves, and be secure from 

 the pernicious effects of the atmosphere. The surface 

 of the earth is always sterile, however, when it shows a 

 continuity of naked rock, uncovered by vegetable mould. 

 The cultivation of fields and woods, and even the rear- 

 ing of cattle, cannot therefore find scope in regions which 

 are entirely rocky. Abrupt and precipitous mountains 

 being generally in this condition are usually barren ; but 

 in plains and on declivities, a bare rocky surface is much 

 less frequently the cause of sterility than an unfavourable 

 proportion of mould. Some rocky and moderately ele- 

 vated regions also occur, more or less destitute of vege- 

 table mould, whose sterility depends upon volcanic caus- 

 es. Iceland, for example, affords cases of this descrip, 



