LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 33 



respect from the pyroxenes described. Nevertheless the exact de- 

 terminations of solidus and liquidus affords a sound foundation for 

 the discussion of zoning in the plagioclases. 



When the cooKng is at such a rate that zoning results, the 

 composition of the liquid is continually offset toward pure albite. 

 There is a certain definite rate of cooling which gives maximal 

 zoning, in which case the last minute quantity of liquid as well as 

 the outer zone of the crystal has the composition of pure albite, 

 whatever the total composition of the original material may have 

 been. With a somewhat quicker rate of cooling, the range of zoning 

 is not so great owing to a moderate degree of undercooling, and when 

 the undercooling "s very great there is no zoning at all. With a 

 rate of cooling somewhat slower than that which gives maximal 

 zoning, the range of zoning is, again, not so great, owing to the 

 opportunity afforded for partial adjustment of composition between 

 the various zones, and when the cooling is extremely slow there is 

 no zoning at all on account of perfect adjustment. Since zoning 

 is absent in this latter case of extremely slow cooling, there is no 

 continuous offsetting in the composition of the liquid in the direc- 

 tion of pure albite from that cause, but the great opportunity for 

 the settling of crystals afforded by slow cooling enters into the 

 problem with the tendency to produce the same result. It is 

 therefore clear that the tendency toward the continuous offsetting 

 in the composition of the liquid increases with slow cooling. 



' CRYSTALLIZATION IN THE SYSTEM DIOPSIDE-ANORTHITE-ALBITE 



It was with the purpose of showing the manner in which the 

 properties of plagioclase mixtures are carried into polycomponent 

 systems that the investigation of the ternary system diopside- 

 plagioclase was undertaken, and fortunately definite results were 

 obtained. 



Fig. 6 is the equilibrium diagram. All Hquids whose composi- 

 tion is represented by points in the field BCED are capable of 

 existing in equilibrium with plagioclase, and all hquids in the field 

 AED may exist in equilibrium with diopside; consequently all 

 liquids represented by points lying along the boundary curve DE 

 can exist in equilibrium with both plagioclase and diopside. In 



