CRYSTALLIZATION-DIFFERENTIATION IN MAGMAS 395 



theory of petrogenesis it was only after these data had been applied 

 to the problems of the subalkaline rocks that a possible solution of 

 the alkaline rocks became apparent. In short, then, the conclusions 

 arrived at in the case of the subalkaline rocks represent but a 

 short extrapolation from the experimental data, wjiereas in the 

 case of the alkaline rocks the extrapolation is a long one and 

 correspondingly subject to error. While there is no reason at 

 present for receding from the posi|;ion taken regarding the genesis 

 of alkaline types, it is quite possible that the position might prove 

 wholly untenable without any necessary . doubt being cast on the 

 strength of the hypothesis that the subalkaline rocks, at least, are 

 derived by processes of differentiation in which fractional crystalli- 

 zation is the primary control. 



The writer's hypothesis, then, refers the diversity of igneous 

 rocks to the differentiation of basic magma through a spontaneous 

 power resident within the magma and resulting from its own 

 crystallization. Daly's hypothesis likewise assumes original ighe- 

 ous material of a basic nature, but considers that it has little 

 power of spontaneous differentiation and that it acquires a signifi- 

 cant tendency to differentiate only when equilibrium is upset by the 

 assimilation of foreign material. For each great family of igneous 

 rocks a particular type of foreign material has been assumed to be 

 the special agent. To Daly assimilation is all-important, to the 

 writer it is a mere adventitious circumstance. There is nothing, 

 however, in the writer's hypothesis that justifies Daly's statement 

 that he assumes differentiation to affect only purely juvenile 

 magma without foreign contamination."^ Daly makes this state- 

 ment in his discussion of the writer's definition of differentiation, 

 in which discussion he directs his attention more toward the form 

 than toward the substance. In another part of his paper (pp. 88 

 and 89) the writer describes his conception of the differentiation 

 of a magma that has suffered foreign contamination. The inter- 

 pretation that Daly puts upon the definition, while perhaps not an 

 impossible one in the light of its wording, is rather surprising in 

 anyone who has read the whole paper as carefully as Daly has 

 evidently done. The phrase "without foreign contamination" 



^Joiir. GeoL, XXVI (1918), 117. 



