VOLUME XXX NUMBER 3 



THE 



JOURNAL OF GEOLOGY 



April-May ig22 



THE REACTION PRINCIPLE IN PETROGENESIS 



N. L. BOWEN 

 Geophysical Laboratory, Carnegie Institution of Washington 



INTRODUCTION 



It is now many years since petrologists first began to think of 

 the crystalKzation of a molten magma in terms of the physico- 

 chemical principles governing the behavior of solutions. In the 

 study of ordinary solutions a condition frequently found was that 

 known as the eutectic relation. In the simple case of two com- 

 ponents, each lowered the melting-point of the other to the temper- 

 ature of the eutectic point, at which temperature both solids 

 separated side by side from a solution of fijced composition, the 

 eutectic mixture. This case and the analogous condition in sys- 

 tems of more components are now so famiHar to petrologists 

 that they need not here be enlarged upon. The concept of the 

 eutectic was early seized upon by petrologists and has been one of 

 great utiHty in petrogenic theory. It accounted for the low 

 melting temperatures of mixtures of rninerals that are individually 

 highly refractory. It threw hght on some of the factors governing 

 the separation of minerals from their mutual solution. But most 

 of all, it stimulated the tendency to think of magmas in the light 

 of the laws of solutions, or, better, of phase equilibrium, and 

 encouraged experimental research whose expected result was the 

 location of the composition of the eutectics for chosen mineral 



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