54 CHEMICAL DISINTEGRATION. 



cause the rocks of which they are the particles are 

 so various. Thus the stratum may have been of 

 sandstone, when the soil produced will of course 

 be sandy ; if of limestone, then the calcareous ele- 

 ment will prevail ; if of slate, then clay. The min- 

 erals which are generally found in the rocks are 

 quartz, felspar, mica, amphibolite, pyroxenite, talc, 

 serpentine, and diallage. These, according to 

 Boussingault, are formed of metals which enter into 

 the structure of plants. Thus quartz is almost 

 pure silica, or sand. 



Sir Humphrey Davy observed, with regard to the 

 chemical causes of disintegration : " The manner in 

 which rocks are converted into soil may be easily 

 conceived by referring to the instance of soft or 

 porcelain granite. This substance consists of three 

 ingredients — quartz, felspar, and mica. The quartz 

 is almost pure silicious earth in a crystalline form. 

 The felspar and mica are very compounded sub- 

 stances ; both contain silica, allumina, and oxide 

 of iron. In the felspar there is usually found lime 

 and potassa ; in the mica, lime and magnesia. 

 When a granite rock of this kind has been exposed 

 to the influence of air and water, the lime and po- 

 tassa contained in its constituent parts are acted 

 upon by water, or carbonic acid ; and the oxide of 

 iron, which is generally in its least oxidized state, 

 tends to combine with more oxygen. The conse- 

 quence is that the felspar decomposes, and also the 



