10 



THE ORIGIN OF SOILS 



[chap. 



The undissolved residue of A was a white, of B a 

 yellow, clay containing fragments ot quartz and flakes 

 of mica. 



The following analyses show the change that takes 

 place in passing from orthoclase felspar to kaolin; in 

 the third column the analysis of kaolin is recalculated 

 to show what arises from 100 parts of felspar, on the 

 assumption that none of the alumina is removed by 

 solution : — 



% Mica is essentially a double silicate of alumina and 

 potash, with some oxide ol iron : the potash being 

 replaced by magnesia in black mica or biotite. Mica 

 splits up into minute flakes as the rock weathers, but 

 these flakes are fairly resistent to chemical change, and 

 may be detected in most sands and sandstones. Ulti- 

 mately, however, they pass into hydrated silicates of 



