174 
cement of the stone, forms a fine clay : the mica 
partially decomposed mixes with it as sand ; and 
the undecomposed quartz appears as gravel, or 
sand of different degrees of fineness. 
As soon as the smallest layer of earth is formed 
on the surface of a rock, the seeds of lichens, 
mosses, and other imperfect vegetables which are 
constantly floating in the atmosphere, and which 
have made it their resting-place, begin to vegetate ; 
their death, decomposition, and decay, afford a 
certain quantity of organizable matter, which mixes 
with the earthy materials of the rock ; in this im- 
proved soil more perfect plants are capable of sub- 
sisting ; these in their turn absorb nourishment 
from water and the atmosphere ; and after perishing, 
afford new materials to those already provided : the 
decomposition of the rock still continues ; and at 
length by such slow and gradual processes, a soil 
is formed in which even forest trees can fix their 
roots, and which is fitted to reward the labours of 
the cultivator. 
In instances where successive generations of 
vegetables have grown upon a soil, unless part of 
their produce has been carried off by man, or con- 
sumed by animals, the vegetable matter increases 
in such a proportion, that the soil approaches to a 
peat in its nature ; and if in a situation where it 
can receive water from a higher district, it becomes 
spongy, and permeated with that fluid, and is gra- 
dually rendered incapable of supporting the nobler 
classes of vegetables. 
Many peat-mosses seem to have been formed by 
the destruction of forests, in consequence of the 
