MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 615 



As to the dislocation of the eutectic by uniform pressure, H. W. 

 Bakhuis Rooreboom^ has stated that in this case also the formula 

 of Clausius-Chapeyron is applicable. The rise of temperature of the 

 binary (and the ternary) eutectic of the rock-forming minerals 

 by pressure may consequently be measured by the same small 

 measures that have just been indicated for the minerals themselves. 



Moreover, it is to be noted that the tangent to the melting curve 

 near the origo (respectively 100 per cent a and 100 per cent b) is 

 determined by the factors: T 

 (absolute temperature), q (melt- 

 ing heat), and the electrolytic 

 dissociation which three factors 

 are but little dislocated by pres- 

 sure, and finally the molecular 

 weight which is constant if no 

 poljrmerisation takes place. In 

 other words, the melting curve 

 in a binary system (Fig. 50) near 

 the origo will, at high pressure, 

 take a course parallel to that of 

 the melting curve at low pressure. 

 The further course of the melting 

 curve is also essentially deter- 

 mined by the constants just 

 mentioned, which are generally 

 very little dislocated by pressure. 

 This relates to the general experience that pressure has but little 

 influence on the solubility of the mutual solutions, provided that 

 no vapor phase is present. 



The contents of a and h at the intersecting point of the curves, 

 in other words the composition of the binary eutectic, (Ep, at low 

 pressure, and Epn at high pressure) will accordingly be but very 

 little dislocated by pressure. And this accords with the fact that 

 the temperature of Epn, as just stated, is only very little higher than 

 that of Ep,. This reasoning can also be extended so as to be applied 

 to the ternary and still more complex eutectics. 



' Heterogene Gleichgewichte, II (1904), p. 715. 



a 



Fig. 50. — Illustrating the unessential 

 dislocation of the composition of the 

 binary eutectic by uniform pressure. 



Tp,. Tp^ = melting point and Ep^, Ep^ = 

 binary eutectic by respectively low and 

 high pressure. 



