198 N. L. BOW EN 



If a reaction of the kind suggested (involving quartz, feldspar, 

 and mica molecules) is a definite feature of pegmatitic granite it is 

 possible that failure of reaction may produce a Hquid deficient in 

 silica, just as such a Kquid is produced in the system above. From 

 this Kquid feldspathoid may be deposited instead of some of the 

 feldspar. Thus it is possible that alkalic rocks may, in some cases, 

 be formed as a result of the reaction relation among the minerals 

 noted. Foye has noted a very intimate relation between the granite 

 pegmatites and nephehte syenite of Haliburton, Ontario.^ 



Note. — Since the foregoing was written it has been demonstrated by Morey and 

 Bowen that leucite and orthoclase constitute a reaction pair with quartz as a released 

 mineral. The importance of the reaction principle in connection with the origin of 

 some alkaline rocks is confirmed by these facts but the full significance cannot be 

 discussed here. 



SUMMARY 



Petrogenic theory has passed beyond the stage where the con- 

 ception of eutectics can longer be regarded as of any considerable 

 service. Experimental investigations and the study of the rocks 

 themselves, in the Kght of such investigations have made it clear 

 that the eutectic relation is unimportant but that another relation 

 between hquid and crystal phases, here called the reaction relation, 

 is of fundamental significance. The ordinary soHd solution series 

 such as the plagioclases may be regarded as a continuous reaction 

 series because during crystallization each member is produced 

 from an earHer member by reaction with the Kquid, the variation 

 of composition being continuous. There are also discontinuous 

 reaction series exhibiting related characters but with discontinuous 

 changes of compositioQ. The series oKvine-pyroxene-amphibole- 

 mica is a prominent example among the rock-forming minerals. 



On the basis of these considerations the minerals making up 

 the rocks of an igneous sequence can be arranged as reaction series 

 and it is the existence of such series that controls the crystalKza- 

 tion and differentiation of the rocks of the sequence. Even the 

 graphic structure, usually regarded as a eutectic structure, is prob- 

 ably to be considered the result of reaction between the phases in 

 many examples. 



January, 1922 



' Amer. Jour. Sci., Vol. XL (1915), p. 436. 



