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JOURNAL OF GEOLOGY—SUPPLEMENT 



Fig. 7 the temperature relations of the fusion surface are indicated 

 by means of isotherms or temperature contours. The isotherms 

 indicate the shape of the fusion surface in the same manner that 

 elevation contours represent the shape of the surface of the ground 

 on a topographic map, the contour interval being a certain number 

 of degrees instead of a certain number of feet. Any point on Fig. 7 



ANORTHITE 



Fig. 6.— Field diagram of the system: diopside, plagioclase 



represents, therefore, not only a definite composition, but also a 

 definite temperature. Any point within the plagioclase field 

 represents the composition of a liquid which can exist in equilibrium 

 with plagioclase and at the same time represents the temperature 

 at which that liquid becomes just saturated with plagioclase. 



The crystallization of typical mixtures may now be considered. 

 On account of the occurrence of a mix-crystal series it is necessary 

 to distinguish, as was done in the system involving the pyroxene 



