﻿488 DR. WHITMAN CROSS ON THE NATURAL [Aug. I9IO, 



eutectic composition. But the most typical porphyries of my 

 experience, the monzonitic rocks of laccoliths, sills, and dykes in 

 the Rocky Mountain and Plateau districts of the United States, 

 possess a ground-mass consisting chiefly or exclusively of quartz 

 and orthoclase, while abundant plagioclase occurs in phenocrysts. 

 Surely plagioclase (Ab-fAn) entered into the eutectic of these 

 magmas. 



I believe it to be a fact of experience that the consolidation- 

 products of any given magma exhibit, under the various conditions 

 of different occurrences, all possible proportions of ground-mass 

 to phenocrysts. 



The view that certain textures, such as the graphic, the spherulitic, 

 and the felsitic, are characteristically eutectic seems to me incorrect. 

 Intergrowth, whether graphic or spherulitic, may mean simul- 

 taneous crystallization, but I question the necessity for eutectic 

 proportions. In any case these textures are due to physical con- 

 ditions, since it must be admitted that they are by r no means 

 always developed in the presence of the eutectic. 



Conclusion. — In view of the care with which a host of 

 observers have studied all details of rocks, the world over, it does 

 not seem to me rash to assert that the eutectic has left no stamp 

 on the consolidated magma, the rock, with possibly a few excep- 

 tions. If we consider the mineral and textural characters of the 

 igneous rock it appears that, in the first place, the composition of 

 the solution as a whole, and, in the second place, the conditions 

 influencing crystallization, such as supersaturation, mass action, 

 rate of cooling, escape of volatile constituents, and viscosity of the 

 magma, have had far more influence upon the rock than the cha- 

 racter of the eutectic. The latter has itself split up into minerals 

 Avhich are not different in their rock-making qualities from those 

 molecules that were in excess in the magmatic solution, and they 

 must be in part identical. In many cases escape of mineralizers, 

 fractional crystallization, and other influences seem likely to have 

 so changed the character of the assumed eutectic that the magma 

 may have had several different eutectics in the course of crystal- 

 lization. 



The propositions of Yogt do not seem to me to have significance 

 as bearing on systematic petrography-. He has gone far from 

 the base of real knowledge of rock eutectics, and while his brilliant 

 original studies and theoretical discussions have done much to 

 advance the application of physical chemistry to the study of 

 magmas, they do not appear to have indicated a future practical 

 factor for petrographic system. A classification by eutectics 

 may in the future be realized, but it seems inevitable 

 that it must be a classification for a special interest, 

 not for the general science of petrography. 



