428 We Teh, be OCI 
commences to crystallize, if subcooling is left out of consideration, 
at atemperature of 1275°. After the separation of a certain amount 
of diopside the melting mass arrives at a point (”, ca. 38 per cent 
Diops:62 per cent Ab,.An;, and at a temperature of 1235°) on the 
eutectic boundary curve. Then a simultaneous crystallization of 
diopside and plagioclase commences, the latter in the beginning with 
a composition Ab,.Ango, but on continued cooling with continually 
less An. On the presumption of a complete equilibrium between 
the solid and liquid phases, accordingly between the already 
crystallized plagioclase and Ab+An in the solution, the composition 
of the crystallized plagioclase is continuously displaced. The 
quantity of the liquid grows continually less by continuous crystal- 
lization. The last remnant of liquid is spent at a point (O on 
Fig. 6) 23 Diops:60 Ab:17 An, at the melting-point 1200° and 
with a separation of a minimal quantity of plagioclase Ab;.Ansp. 
If, on the other hand, we choose a melt of 15 per cent CaMgsSi,0. 
and 85 per cent Ab,oAn;o.—consequently with a surplus of Ab+An— 
plagioclase of the composition Ab,.Ang, commences crystallizing 
at the temperature 1375° (see point F on Fig. 6). On continuous 
cooling at first only plagioclase crystallizes. On the presumption 
of a complete equilibrium the composition of the already separated 
plagioclase is continuously changed. In this manner a plagioclase 
Ab,An, appears at 1300°. The eutectic boundary curve is reached 
at G, at a temperature of 1216°. Now a simultaneous crystalliza- 
tion of diopside and plagioclase commences, as the Ab: An relation 
in the separated plagioclase little by little is displaced. At 1200° 
the last remnant of liquid is spent, the entire mass of plagioclase 
having the composition Ab;.Anso. 
With lacking equilibrium the first separated plagioclase remains 
unchanged. By the continuous separation of relatively An-rich 
plagioclase, the quantity of Ab in the solution increases continually, 
and even more strongly than by the equilibrium. In this manner, 
by the simultaneous crystallization of diopside and plagioclase 
along the eutectic boundary-line we here at last obtain a plagioclase 
with a very high percentage of Ab. 
Bowen emphasizes—and rightly—that in a ternary system 
Ab:An:Diops (as in the analogous system Ab:An:Qu) there is 
