LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 37 



If the composition of the original hquid is that of the point N, 

 which consists of 70 per cent plagioclase AbiAni and 30 per cent 

 diopside, crystallization begins with the separation of bytownite 

 (AbiAn4 approximately) at 1302°. As the temperature falls the 

 composition of the liquid changes, along the curve NP, plagioclase 

 continues to separate, and its composition changes until at 1230°, 

 when the liquid has the composition P, the feldspar has the com- 

 position i?(AbiAn3). With further cooling, the composition of the 

 liquid changes along the boundary curve, diopside separating as 

 well, and, as in the preceding case, the last of the liquid disappears 

 at 1200°, when it has the composition M, and the plagioclase 

 crystals have changed gradually to AbiAui. The whole mass now 

 consists of 70 per cent plagioclase AbiAui and 30 per cent diopside. 



The two mixtures F and N are merely random mixtures of 

 AbiAui and diopside on opposite sides of the boundary curve. All 

 mixtures of AbiAni and diopside, therefore, become completely 

 crystallized at 1200°, when perfect equilibrium is obtained, and the 

 last minute quantity of liquid has the composition M, which, 

 expressed mineralogically, is 78 per cent Ab4Ani and 22 per cent 

 diopside. 



All mixtures of diopside with AbiAua become completely 

 crystallized at 1220°, the last of the liquid having the composition 

 K. Mixtures of AbaAui and diopside are completely crystalline 

 at 1178° and the last of the liquid has the composition S. Thus a 

 mixture of diopside with any plagioclase has a temperature of 

 final consolidation different from that of a mixture of diopside 

 with any other plagioclase, and to speak of a eutectic between 

 diopside and plagioclase is to use a meaningless phrase.^ "The 

 eutectic of plagioclase and pyroxene" is even more emphatically 

 meaningless when the pyroxene referred to is a complex mix-crystal 

 such as augite. 



Crystallization with great undercooling. — -The same final product 

 will be obtained in each of the foregoing cases if the liquid is greatly 



' Even the definite temperature of final consolidation of mixtures containing a 

 certain definite plagioclase is not properly to be called a eutectic. Thus 1 200° is not the 

 eutectic temperature of AbiAni and diopside, and the hquid M is obviously not the 

 eutectic mixture of AbiAni and diopside. 



