LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 29 



tridymite also separates and the whole crystallizes to a mixture of 

 the pyroxene, dino-enstatite, anorthite, and silica. Thus the sinking 

 of crystals has brought about a partly different assemblage of 

 minerals in the upper and lower layers, a difference in temperature 

 of final consolidation and in the composition of the final liquid 

 of these layers. It is evident that even if the cooKng were too 



Fig. 4. — Equilibrium diagram of the system: anorthite, forsterite, silica, after 

 Andersen, Am. Jour. Sci. (4), XXXIX (1915), 437- 



quick to permit of any considerable sinking of crystals, the same 

 consequence to the composition of the final liquid would result from 

 the formation of a reaction rim of clino-enstatite about the forsterite 

 crystals and their protection thereby from further resorption by the 

 liquid. 



If the composition of the original liquid were that of the point z, 

 crystallization of the perfect equihbrium type and also crystallization 



