142 HOW CROPS FEED. 
This direct action of the living plant is 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 contain oxalate of lime to the 
extent of half their weight (Braconnot). 
According to Gveppert, the hard, fine-grained rock of the 
Zobtenberg, a mountain of Silesia, is in all cases softened at 
its surface where covered with lichens (Acarospora smar- 
agdula, Imbricaria olivacea, 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 covering 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. 
§ 1. 
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 serve to give a certain degree 
of precision to language, and enable us to discriminate in 
some measure the different kinds of soils, which offer 
great diversity in origin, composition, external characters, 
* See, also, p. 186. 
