THE REACTION PRINCIPLE IN PETROGENESIS 195 



nothing is known as to the details of the amphiboles or the biotites 

 as reaction series, and this means not merely their possible range 

 of composition but also how those compositions are arranged in the 

 series. The continuous reaction series of the plagioclases occupies 

 a more conspicuous place than the other continuous reaction 

 series, not merely because we know more about it, but because, as 

 already pointed out, it enters into the rock series through a much 

 wider range of conditions. Of the plagioclases we require to know 

 how the potash feldspar enters into their make-up and, in the present 

 connection particularly, at what stage in its concentration the 

 potash molecule must appear as a separate phase. We require to 

 know, too, the exact composition of pyroxene that forms by reac- 

 tion from olivine of a certain composition, the exact composition 

 of amphibole that forms from a certain pyroxene, and so on. 

 These are questions on which much light may be shed by systematic 

 equilibrium studies in synthetic minerals and by careful separation 

 and analysis of associated minerals in various rock types. Studies 

 of this kind are now being carried on at this Laboratory by Aurous- 

 seau on oHvines, by Washington and Merwin on pyroxenes, and 

 by Buddington on melilites. Eskola points out the importance 

 of such work on the natural minerals, referring to it under the 

 special name, facies petrology.' The results of these and kindred 

 investigations may be profitably regarded in the light of their 

 bearing on the reaction principle, for in their manifold details they 

 are likely to prove excessively wearisome unless considered in 

 relation to some such co-ordinating principle. 



REACTION SERIES AND MAGMATIC DIFFERENTIATION 



As the information gained from such studies of reaction series 

 accumulates, our knowledge of igneous rock differentiation should 

 increase conspicuously, for the reaction principle is the very life- 

 principle of differentiation. We have seen, in our examination of 

 the simple systems used to illustrate reaction series, how the exist- 

 ence of the reaction relation lends a flexibility to the behavior of a 

 cooling liquid, renders it capable of giving different products 



^P. Eskola, "The Eclogites of Norway," Videnskapsselskapets Skrifter I. Math. 

 Natiirv., Klasse No. 8 (1921), p. 6. 



