i8o 



N. L. BOWEN 



equilibrium diagram of this system is given in Figure 2. Again it 

 is a familiar diagram that has been fully discussed in other places/ 

 but certain features of it require emphasis in the present connection. 

 A mixture of composition F (diopside 50 per cent, AbjAni 50 

 per cent) begins to crystalKze at 1275° with separation of diopside. 

 As the temperature falls the crystals merely increase in amount. 



ALBITE I «>^", •- R H ANORTHITE 



Fig. 2. — Crystallization diagram in the system, diopside-anorthite-albite 



At 1235°, when the liquid has the composition G, plagioclase of 

 composition AbiAn4 begins to separate and thereafter the change 

 of composition of the hquid is represented by the boundary curve 

 ED. And as crystallization proceeds along the boundary, the same 

 reaction relation exists between the plagioclase crystals and the 

 liquid as in the binary system. Vogt would call the curve ED a 

 eutectic boundary curve or eutectic line and, to be sure, it has some 

 properties in common with such a Hne.^ It marks the composition of 



' N. L. Bowen, op. cit., Vol. XL (1915), p. 175. 



= J. H. L. Vogt, Journal of Geology, Vol. XXIX (1921), p. 429. 



