142 UOW CROPS FEED. 



This direct action of the living plant h probably ex- 

 erted by the lichens, which, as has been already stated, 

 grow upon the smooth surface of the rock itself Many 

 of the lichens are known to contahi oxalate of lime to the 

 extent of half their weight (Braconnot). 



According to Goeppert, the hard, fine-grained rock of the 

 Zobtenberg, a mountain of Silesia, is in all cases softened at 

 its surface Avhere covered with lichens {Acarospora smar- 

 agdula, Imhricaria oHvacea, etc.), while the bare rock, 

 closely adjacent, is so hard as to resist the knife. On the 

 Schwalbenstein, near Glatz, in Silesia, at a height of 4,500 

 feet, the granite is disintegrated under a covei-ing of li- 

 chens, the feldspar being converted into kaolin or washed 

 away, only the grains of quartz and mica remaining unal- 

 tered.* 



CHAPTER III. 



KINDS OF SOILS— THEIR DEFINITION AND CLASSIFI- 

 CATION. 



DISTINCTION OF SOILS BASED UPON THE MODE OF THEIR 

 FORMATION OR DEPOSITION. 



The foregoing considerations of the origin of soils intro- 

 duce us appropriately to the study of soils themselves. 

 In the next place we may profitably recount those defini- 

 tions and distinctions that servo to give a certahi degree 

 of precision to language, and enable us to discriminate in 

 some measure the different kinds of soils, which offer 

 great diversity in origin, comjiosition, external characteis, 



♦ See, also, p. 136, 



