132 



HOW CROPS FEED. 



bonic acid, oxygen, and the salts held in solution by the 

 atmospheric waters, is expressed by the word veathering. 

 Tliis term may likewise include the action of frost. 



When rocks weather, they are decomposed or dissolved, 

 and new compounds, or new forms of tlie 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 cliief ingredient of all the granitic, horn- 

 blendic, and 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 yarieti«s, 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. 



h. 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 exceeding!}^ durable. 

 The decomposition results in completely breaking up the 

 liard, 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, ^Y/o/m/^e, ahydrated silicate of a]umina,(the 

 analysis of which has been given already, p. 113,) mixed 



