THE REACTION PRINCIPLE IN PETROGENESIS 185 



liquid that fails to crystallize at all. The formation of this liquid 

 under such conditions is important in connection with the abundance 

 of analogous liquid in certain types of magma, say, the granitic. 



RELEASED MINERALS 



A feature of reaction series that is important in its consequences 

 is the fact that a compound, which otherwise would not be formed 

 from a given liquid at all, may, as a result of failure of complete 

 reaction, be stored up in the liquid and appear as a mineral at 

 some later stage. Such a mineral may be regarded as released 

 because of the existence of the reaction relation and it is at the 

 same time complementary in composition to the mineral disappear- 

 ing during reaction. Thus in the case of the reaction pair Mg2Si04- 

 MgSiOj free silica is released as a result of failure of complete 

 reaction and it is complementary to Mg2Si04. 



THE EFFECT OF THE REACTION SERIES ON THE 

 ORDER OF SEPARATION 



We have now examined several examples of the manner in which 

 reaction series may render the course of crystallization responsive 

 to external conditions. In another important particular the exist- 

 ence of the reaction series, continuous and discontinuous, causes 

 the process of crystallization to depart from that obtaining in the 

 eutectic system. In the crystalKzation of the plagioclase feld- 

 spars a plagioclase always separates before any other plagioclase 

 that is less calcic. There is no such thing as the separation of 

 calcic plagioclase first from mixtures rich in calcic plagioclase and 

 of sodic plagioclase first (followed by calcic plagioclase) from 

 mixtures rich in sodic plagioclase, as there would be in the eutectic 

 system. And so with the reaction pair and the discontinuous 

 reaction series the higher member of the series always separates 

 before the lower, if at all. We do not have a condition in mixtures 

 of forsterite and chnoenstatite such that forsterite separates first 

 from mixtures rich in forsterite and chnoenstatite first in mixtures 

 rich in chnoenstatite as there would be in eutectic mixtures. On 

 the contrary, forsterite, however small in amount, always separates 

 first. 



