CHEMICOMINERAL RELATIONSHIPS IN ROCKS 225 



except varying percentages of silica the resulting rocks if wholly 

 crystallized would consist of orthosilcate of sodium and aluminium, 

 nephelite ; this with polysilicate of the same elements, albite ; 

 albite alone ; or albite and quartz, according as silica is lower or 

 higher. We may say the rocks would lie on the curved line 

 QAN. The point Q is where pure quartz occurs ; and the point 

 A where pure albite occurs ; and TV where soda-nephelite occurs. 

 Pure albite rock would occur at A. Any rock on the line 

 between A and TV would consist of albite and soda-nephelite 

 in proportions varying from one extreme to the other. Any 

 rock on the line between A and Q would consist of albite and 

 quartz in proportions varying in the same manner. 



From A as a point of departure let us consider what must be 

 the composition of soda-alumina rocks not occurring on the line 

 QAN. If we consider magmas with constant alkali-silica ratios, 

 namely .166, it is evident that proceeding from A horizontally, 

 as the silica percentage becomes less, the soda and alumina 

 must also become less, consequently the sum of these constitu- 

 ents will not be 100, which is always the total of the percentages 

 in any case. There must therefore be other chemical constitu- 

 ents besides Na 3 0, A1 3 3 and SiO s . The same is true if we 

 proceed from A vertically downwards, that is, if we consider 

 magmas having the same silica percentage, namely 68.7, for 

 with decreasing alkali-silica ratios there will be less soda and 

 alumina, and the sum of Na 3 0, Al 3 O s , Si0 3 will notbe 100. The 

 same will be true if we proceed in any direction from A below 

 the line QAN. 



All rocks except those occurring upon this line must contain 

 other constituents than Na s O, A1 3 3 , Si0 3 . These may be CaO, 

 MgO, FeO, Fe 3 3 (potash being excluded by the initial assump- 

 tion). Let us assume them to be either CaO, MgO, FeO, 

 singly or together, and that they occur as silicates. The case 

 of FeO.Fe 3 3 , magnetite, will be considered subsequently. 

 It having been assumed that alumina is present only in propor- 

 tions to satisfy soda, the possibility of the formation of an alum- 

 inous molecule with these constituents will be excluded for the 



