42 INFLUENCE OP ROCKS UPON THE FERTILITY OP SOIL, &C. 
surface and aiding by its decomposition in the formation of the 
soil, communicates to it a very high degree of fertility. 
2. Susceptibility of Disintegration and Surface. — A wide 
difference in their susceptibility of disintegration, is observed 
amongst rocks bearing the same name. Some varieties of granite 
lose their consistency and are converted to a great depth into a 
mass of coarse gravel, whilst others undergo but inconsiderable 
changes, though exposed to the action of the weather for ages. 
This rock is supposed to be attacked by destroying agents, through 
the medium of the potassa entering into the composition of its 
feldspar. It is not always the softest rock that is the most liable 
to decay. Soapstone, though so soft as to be easily cut with a 
knife, is amongst the most indestructible of the rocks. When a 
rock formation, by reason either of its composition or of the man- 
ner in which its integrant panicles are united, is not affected by 
those causes which change the aspect and condition of other por- 
tions of the earth's crust, it is evident that it may fail for many 
ages of covering itself with such a coating of soil as is adequate to 
the demands of agriculture ; as also that if its surface be mountain- 
ous and broken, the soil may be carried away by the rain as 
rapidly as it is formed. The susceptibility of disintegration, and 
surface, of the strata have therefore an influence, but incomparably 
less in most cases than their composition, upon the fertility of the 
soils they form. 
1. The primitive rocks are said to produce by their decompo- 
sition a soil of very moderate fertility. Granite is also stated to 
be inferior in regard to the soil it forms to gneiss, and gneiss to 
mica slate. In many cases all three of the circumstances upon 
which the formation of a good soil depends, are wanting in this 
class. Silica often predominates in their composition, so that 
there is not a due proportion of alumina; it is seldom that they 
contain any lime, their disintegration proceeds slowly, and occupy- 
ing as they do the crests of mountains, they are liable to be washed 
away as soon as the attraction of cohesion that united their par- 
ticles, is destroyed. They abound in pure springs of excellent 
water, and with few exceptions are occupied by a vigorous and 
healthy though sparse population. 
A large part of the good land in North Carolina has been 
produced by the decomposition of primitive rocks. The most 
ancient primitive formation, extending along the bases of the 
western mountains and to a considerable distance east of them, has 
good land on the banks of the creeks and rivers, but the largest 
bodies of soil susceptible of cultivation, have proceeded from a for- 
mation of recent primitive rocks, stretching through the middle 
counties, and the tracts where hornblende predominates in their 
composition, as in Cabarrus and Mecklenberg, are in general supe- 
rior to the rest. 
2. The transition rocks being composed of particles that are 
not associated in the close and intimate union produced by chemi- 
