i8 JOURNAL OF GEOLOGY— SUPPLEMENT 



chilled, it crystallized as a normal plagioclase-pyroxene diabase 

 without quartz. On the other hand, large bodies usually show 

 micropegmatitic interstices and often a similar saHc differentiate. 

 This contrast between the larger and the smaller bodies has led 

 some petrologists to the opinion that the more slowly cooled, large 

 bodies had an opportunity denied the quickly cooled bodies — ■ 

 the opportunity to assimilate siliceous material, whence the siliceous 

 differentiate. Direct evidence of adequate assimilation is seldom 

 if ever clear; its accompHshment is nearly always inferred from 

 the existence of the acid differentiate. In the following an attempt 

 will be made to show that the distinction often noted between the 

 large and small bodies is solely the result of a difference in the 

 course of crystallization dependent upon the difference in the rate 

 of cooling. The plan will be to discuss the physical chemistry of the 

 crystallization of investigated systems which have a direct bearing 

 on the question and then to apply the facts and principles developed 

 to the crystallization of basaltic magma. It may seem, perhaps, 

 that a discussion in which physical chemistry plays so prominent 

 a part has no place in a geological paper, but it is in reality one 

 of the many phases of that subject which the petrologist must 

 master if a foundation is desired on which a discussion of the crystal- 

 lization of igneous magmas may be based. 



THE SYSTEM DIOPSIDE-FORSTERITE-SILICA 



In a recent paper the writer has pubKshed an investigation of 

 the system diopside-forsterite-silica, in which the comparative 

 effects of slow and of quick cooling were discussed on the basis of 

 definite experimental results. The discussion has sufficient impor- 

 tance in the present connection to warrant its partial repetition 

 here. For fuller details the original paper may be consulted.^ 



Fig. I is an ordinary ternary composition diagram, composition 

 being plotted on triangular co-ordinates. Any point within the 

 triangle represents a definite composition, definite relative pro- 

 portions of the three components. The area of the triangle is 

 divided into four fields by the boundary curves, BFA, etc. The 

 field of forsterite is such that it contains all the points representing 



^ Am. Jour. Sci. (4), XXXVIII (1914), 207. 



