MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 649 
The substances most easily soluble in acids generally have the 
greatest power of reaction. Therefore it is obvious that, as a 
general rule, the reaction rims should increase according to the 
amount of An contained in the plagioclase, as the solubility in 
acids of the plagioclase increases with its contents of An—and 
in plagioclase more basic than Ab,An, the solubility increases 
obviously. Both olivine and basic plagioclase are much more 
easily soluble in acids than, for instance, the pyroxenes, the horn- 
blendes, the micas, or the acid plagioclases. It is therefore easily 
accounted for that just between olivine and basic plagioclase the 
reaction rims are generally most strongly emphasized. 
The general view here maintained of the reaction in the solid 
phase between olivine and plagioclase may in principle be trans- 
ferred also on the corresponding reaction rims between diallage 
and basic plagioclase, between magnetite or titanomagnetite and 
plagioclase, etc. 
[To be continued] 
