CHEMICAL AGENCY. 
14T 
What is thought to be more durable than granite I 
Yet this rock is rapidly consumed by the decompo- 
sition of its feldspar, effected by carbonic acid gas.; 
a process which is sometimes conspicuous even in 
Britain, but is usually performed in Auvergne, where 
carbonic acid gas issues plentifully from the volcan- 
ic regions." 
Rocks which contain potash and soda are pecu- 
liarly exposed to decomposition, from the fact that 
these substances have a strong affinity for water 
and carbonic acid ; while those that contain iron are 
no less perishable, from its strong attraction for 
oxygen. 
Thus we see that rocks containing iron pyrites or 
sulphuret of iron are constantly crumbling to pieces ; 
the oxygen of the air and water, combining with the 
sulphur, forms a sulphate of iron or copperas, the sul- 
phuric acid of which acts powerfully on the rock, 
causing its rapid decomposition. Indeed, all rocks 
which have for one of their components the protox- 
ides of the metals iron and manganese, soon fall 
into fragments when exposed to the atmosphere, 
from the circumstance that a second portion of oxy- 
gen combines with the metal, causing an increase of 
bulk, and, consequently, a disintegration of the rock. 
As we shall hereafter enter somewhat minutely 
into the subject of the decomposition of rocks, when 
we come to treat of the practical application of ge- 
ological science to economical purposes, our object 
at present will be attained if we succeed in convin- 
cing the reader that rocks are constantly, and often 
rapidly, undergoing decay. 
An examination of the soil in any rocky district 
of country will suffice to convince us that it has 
been derived from the decomposition of the rocks 
in its vicinity. Thus, in a granite district, we find 
the soil made up of grains of quartz, spangles of 
mica, and white or brown looking particles, which 
are feldspar ; and the finer portions, when examined 
