TEE REACTION PRINCIPLE IN PETROGENESIS 187 



over the examples have in most cases dealt with members of com- 

 mon rock-forming groups and the prevalence of reaction series of 

 one kind or the other among the rock-forming silicates is indicated 

 by these few examples. The data are not at hand — and are not 

 likely to be for some time — for a quantitative discussion of reaction 



Fig. 5. — Crystallization diagram of the system, diopside-forsterite-silica. ^, fors- 

 terite; B, silica; C, diopside; H, clinoenstatite, and H-C, clinopyroxene series. 



series in mixtures corresponding to natural magmas. Neverthe- 

 less it is believed that much is to be gained from a qualitative con- 

 sideration of this feature of rock-minerals. 



It should be frankly stated that the existence of the reaction 

 relation between two phases in a simple system is no guarantee 

 of the persistence of an identical relation between them in a more 

 complex system. In the case of the phases olivine and magnesian 

 pyroxene, for which such a relation exists in the binary system 

 Mg0-Si02, it is not unreasonable to expect that the relation might 

 be modified in more complex systems. Actually, however, it is 

 found that the relation persists in all the more complex systems 



