464 ON THE CONNECTION OF GEOLOGY 



quantity, and with more celerity than those which are 

 not disintegrated. The condition of rocks with regard 

 to the attraction of water, affects, in a different manner, 

 the humidity of soil ; for, by this attraction, moisture 

 may as well be abstracted from, as imparted to, the loose 

 earth or soil by which rocks are covered. Part of the 

 moisture which vegetable earth or soil derives from the 

 atmosphere passes into the subjacent mass of rock, but 

 this may again be compensated by evaporation ; on which 

 account the soil of such rocks as have but a small attrac- 

 tion for water usually dries up more readily than soils 

 whose solid substratum attracts and retains the moisture 

 in a greater degree. 



It is probable that the structure of rocks has also a 

 greater, and not less, diversified influence upon the hu- 

 midity of productive soil. Solid rocks, which are not 

 traversed by numerous perpendicular fissures penetrat- 

 ing to a considerable depth, allow the water to remain 

 in the soil ; but columnar and schistose rocks, with per- 

 pendicular fissures, and strata declined from the hori- 

 zontal position, draw off' the water from the soil co- 

 vering their surface, into lower places, where it often 

 re-appears under the form of springs. In these circum- 

 stances, we find a partial explanation of the great diffe- 

 rence between the humidity of soil covering a surface of 

 solid granite, and that lying upon limestone, which is 

 intersected by numerous fissures. Granitic mountains 

 are often furnished with marshes, whereas, on the other 

 hand, the dryness of the soil upon calcareous mountains 

 is generally excessive *, the cause of which phenomenon 



* The dryness depends chiefly, if not entirely, on the fissures or di- 

 visions in the rocky base of the soil : for, in some parts of Sologne in 



