Haplodioritic and Related Magmas. 



Ill 



isotherm. In order to apply this method, then, it is necessary 

 to determine isotherms and isofracts, to hold the desired mix- 

 ture at a measured temperature, quench and determine the 

 refractive index of the glass. The point of intersection of the 

 isotherm of the measured temperature and the isofract of the 

 determined refractive index represents the composition of 

 the liquid. 



Table V. 



Composition 



of glass 





Diopside 



Plagioclase 



Eefractive Index 



100 







1-607 



50 



50 Ab 



1-548 







100 Ab 



1-489 



17-5 



82-5 Ab 1 An i 



1-523 



60 



40 Ab.An, 



1-571 



40 



60 Ab^Aiij 



1-553 



25 



75 Ab^An, 



1-539 







100 Ab 2 An l 



1-517 



50 



50 Ab,An, 



1-569 



30 



70 AbjAn, 



1-553 







100 Ab 1 An l 



1-531 



45 



55 AbjAiij 



1-573 



25 



75 AbjAn^ 



1-560 







100 AbjAn, 



1-545 



60 



40 An 



1-594 







100 An 



1-575 



The measurements of refractive indices, on which the loca- 

 tion of isofracts is based, are given in Table Yand the isofracts 

 are drawn in fig. 12. The refractive indices were determined 

 on glasses of known composition, the glass being compared 

 with immersion liquids until a liquid whose index matched the 

 glass was obtained. The index of the liquid was then deter- 

 mined on the refractometer. The probable error is usually not 

 more than -001 but an error of *002 is possible in some cases. 



If the mixture AbjAn, 85 per cent-diopside 15 per cent (D) 

 is held at 1300° and quenched, the refractive index of the glass 

 is found to be 1-539. Its composition is therefore fixed at the 

 point P, fig. 12. The mixture, Ab 18 An 62 90 per cent-diopside 

 10 per cent (E), held at 1100° gives a glass of refractive index 

 1-561, the composition being, therefore, that of the point R, 

 fig. 12. 



It is important to note also that this determination of the 

 composition of the liquid fixes the composition of the plagio- 

 clase crystals at the same time. Thus the composition of the 

 crystals in the former case is given by joining PD and produc- 

 ing it to Gr which represents the composition of the plagioclase 



Am. Jour. Sci.- 

 12 



-Fourth Series, Vol. XL, No. 236. — August, 1915. 



