LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 21 



the point N is reached, the last of the liquid disappears, the vanish- 

 ing quantity having the composition N, and the pyroxene having 

 the composition P. The whole then consists of pyroxene of compo- 

 sition P and forsterite in the proportions pyroxene :forsterite= 

 MAiMP. 



If the composition of the original hquid had been that of the 

 point P, the separation of forsterite would begin at the temperature 



CaNgSi^Os 



Mg2S!04 



PRIMARY PHASE 

 DETERMINED POINTS • "SIIIke" 



• SILrCA 



Fig. 2. — Temperature-composition diagram of the system: diopside, forsterite, silica 



of the point P, and the course of crystallization would be the same 

 as in the foregoing case except that at the temperature of the 

 point N the last of the forsterite is redissolved and the last of the 

 Hquid is used up at the same instant and the whole consists of 

 pyroxene of composition P. Thus all mixtures of forsterite with 

 pyroxene P, or pyroxene P itself (i.e., any composition represented 



