16 



N. L. Bowen — Genetic Features of 



pear as more and more of other components are added. 

 The writer has investigated the effects of the addition 

 of nephelite to some of these mixtures. The complete 

 quantitative solution of the problem can not be expressed 

 in terms of less than five components, and this is, of 



Boundaries ; 



■Isotherms deter 



</ned; Isotherms by extrapolation , 



! Solid solutions 



o Compounds ; 



Fig. 1. — Equilibrium diagram of the system: CaO-MgO-Si0 2 



(after Ferguson and Merwin). 



course, not possible without a vast amount of preliminary 

 work on the simpler systems. However, an approximate 

 statement is possible ; in other words, a statement of the 

 kinds of mineral phases present at various temperatures 

 may be made without giving the precise composition of 

 the phases of variable composition or the exact relative 

 proportions of the phases. 



