1382 HOW CROPS FEED. 
bonic acid, oxygen, and the salts held in solution by the 
atmospheric waters, is expressed by the word weathering. 
This term may likewise include the action of frost. 
When rocks weather, they are decomposed or dissolved, 
and new compounds, or new forms of the original mat- 
ter, result. The soil is a mixture of broken or pulverized 
rocks, with the products of their alteration by weathering. 
a. Weathering of Quartz Rock.—Quartz (silicic acid), 
as occurring nearly pure in quartzite, and in many sand- 
stones, or as a chief ingredient of all the granitic, horn- 
blendic, an many other rocks, is so exceedingly hard and 
insoluble, that the lifetime of a man is not sufficient for 
the direct observation of any change in it, when it is ex- 
posed to ordinary weathering. It is, in fact, the least 
destructible of the mineral elements of the globe. Never- 
theless, quartz, even when pure, is not absolutely insoluble, 
particularly in water containing alkali carbonates or sili- 
cates. In its less pure varieties, and especially when as- 
sociated with readily decomposable minerals, it is acted 
on more rapidly. The quartz of granitic rocks is usually 
roughened on the surface when it has long been exposed 
to the weather. 
6. The Feldspars weather much more easily than 
quartz, though there are great differences among them. 
The soda and lime feldspars decompose most readily, 
while the potash feldspars are often exceedingly durable. 
The decomposition results in completely breaking up the 
hard, glassy mineral. In its place there remains a white 
or yellowish mass, which is so soft as to admit of crush- 
ing between the fingers, and which, though usually, to the 
naked eye, opaque, and non-crystalline, is often seen, under 
a powerful magnifier, to contain numerous transparent crys- 
talline plates. The mass consists principally of the crys- 
talline mineral, Aaolinite, a hydrated silicate of alumina,(the 
analysis of which has been given already, p. 113,) mixed 
