44 JOURNAL OF GEOLOGY— SUPPLEMENT 



however, give evidence of the existence of certain equiHbrium 

 reactions in the liquid, though leaving in doubt the relative pro- 

 portions of the various compounds taking part in the equilibrium. 



EQUILIBRIA IN MAGMAS INDICATED BY MINERALS PRECIPITATED 



We may now return to the commonly observed association of 

 diabase and its salic differentiate. The principal minerals of the 

 salic differentiate are orthoclase, sodic plagioclase, quartz, and 

 biotite. It appears that special attention has seldom been called to 

 the fact that in the most siliceous rocks (granites, granodiorites, 

 etc.) considerable proportions of the alkalis and of the ferro- 

 magnesian constituents are represented, in the mineral biotite, in 

 their least siliceous combinations.^ A consideration of the equilibria 

 in the magma indicated by this mineral grouping affords an explana- 

 tion of the formation of such a differentiate when diabase magma is 

 slowly cooled, and at the same time leads naturally to a conception 

 of one of the modes of origin of the most important group of the 

 alkaline rocks, the nephelite syenites and their derivatives. 



Imagine a large body of basaltic magma slowly cooled and 

 crystallizing in such a manner that the early-formed pyroxenes and 

 more calcic plagioclases sink slowly in the liquid. The result is 

 a continual enrichment of the part still liquid in alkahne feldspars. 

 There is also a continual enrichment in the volatile constituents 

 such as water, CO2, S, CI, etc. From this liquid are precipitated 

 the minerals of the saHc differentiate including alkaline feldspars, 

 quartz, and biotite. The chemical characters of these minerals give 

 direct evidence of a number of equiHbrium reactions in the liquid. 

 The most important of these reactions involve the breakdown 

 of part of the polysilicate molecules as follows: 



KAlSisOs ±?KAlSi04 +2 SiO^ 

 NaAlSi308±5NaAlSi04 +2 SiO^ 



There must also exist such equilibria as the following: 



NaAlSA +H2O ^HAlSi04 +NaOH 

 KAlSi04 +H2O ^HAlSi04+K0H 



^ See Iddings, Igneous Rocks, I, 133. 



