SOIL-FORMING PROCESSES 25 



The next step is to deduct some general law Avhich can 

 be shown to govern the resistance of these minerals. 

 Such a statement would aid considerably in the making of 

 general deductions regarding weathering. The siliceous 

 content of some of these minerals, taken in the order as 

 above, throws considerable light on this phase : ■— 



Per cent of Si02 Per cent of SiOa 



Quartz 100 Hornblende .... 45 



Orthoclase . , . , 65 Olivine 41 



Plagioclase .... 55 Calcite trace 



Another case might be cited in a comparison of the 

 chemical composition of anorthite, hornblende, and 

 olivine : — 



Anorthite . CaAl2Si208 



Hornblende . Ca(MgFe)2(SiOa) with J^mJf^^^^^^^ 



Olivine . . (MgFe)2Si04 



It is to be noted that immediately as the resistance of a 

 mineral declines, its content of silica decreases and the 

 percentage of the basic constituents increases. Silica 

 and aluminium, then, mark resistance to decay; while 

 calcium, magnesium, sodium, potassium, and iron func- 

 tion in increasing susceptibility to decay. The law of 

 mineral resistance may be formulated as: "The more 

 basic a rock becomes, the more rapid is its decomposi- 

 tion ; and the more acid, the less marked is its decay." ^ 



This general law certainly should apply to rocks that 

 are made up of the minerals listed above. One example 

 will show this clearly. The igneous rocks, as already 

 stated, may be divided into two groups, acid and basic, 



1 Buckman, H. 0. The Formation of Residual Clay. Trans. 

 Anaer. Ceramic Soc, Vol. XIII, p. 362. February, 1911. 



