CHEMICAL AGENCY. 



147 



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 pyritf s 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 cortvia- 

 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 



