MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 613 



Thus the divisor of the formula becomes very great, and con- 

 sequently the melting point of the silicate minerals rises but very 

 little on increasing pressure/ As an illustration I cite a case 

 treated in my publication just quoted: 



For a mineral with melting point = 1 200°, melting heat = 100 caL, 

 density in the solid phase = 3.000 and in the fluid phase = 2.887, 

 vuq.— Vsoi., at the melting point accordingly amounting to 0.013 or 

 3.9 per cent we calculate the melting point at higher pressures: 



We have assumed, in this instance, in accordance with an early 

 examination carried out by C. Barus,^ a percentage value of 3.9 

 per cent, for the difference (at the pressure of one atmosphere) 

 between the specific volume in the fluid and in the solid phase of 

 the melting point, and we assume also the same value at higher pres- 

 sure. 



A series of determinations of glass and of crystalline substance 

 at room-temperature (15° or 20°) show, for rock-forming minerals 

 and for rocks, the following percentage differences of specific 

 volume :^ 



In some cases only about 3 per cent (2.9-3.5 P^^ cent), in most 

 cases between 5 and 8 per cent, exceptionally up to 10.6, 11.4, and 

 in a single case 13.6 per cent."* 



(in his study on plagioclase, loc. cit.) has calculated 104.2 cal. (I foimd 105 cal.=fc2o per 

 cent.) For diopside W. P. White (Amer. Jour. Sci., XXVIII [1909], p. 486, footnote), 

 according to a preliminary determination, states 106 =±=15 cal. (I found 94=^ 15 per cent.) 



' I beg to refer to my statement in Tscherm. Mitt., XXVII (1908). 



^ Phil. Mag. London, XXXV (1893), and U.S. Geol. Surv. Bull. 103. 



3 Most of these statements are grouped from Doelters, Handb. d. Mineralchemie, I 

 (1912), p. 672. 



4 The last named value concerns CaMgSiOaOe with density of the mineral diopside 

 =3.275, and of the glass = 2.830 (Men, White, etc., Amer. Jour, of Sci., XXVII 

 [1909], viiq.— vsol. accordingly=o.3S33-o.3o53=o.o5o. 



