430 N. L. BOWEN 



on this question we find that they speak in no uncertain voice 

 regarding the intimate connection of siHcic with alkahc rocks. 



This association of silicic and alkahc rocks, we have seen reason 

 to believe, is of fundamental significance and is a fact very difficult 

 of explanation on the basis of the limestone-syntectic, desilication 

 hypothesis. Ordinary granites, nordmarkites, and highly siliceous 

 alkaline granites are world-wide associates of nephelite syenites. 



In yet another respect the limestone-syntectic hypothesis 

 lacks support. The proponents of syntexis as a general phenome- 

 non frequently support their claims by pointing to the fact that 

 fragments of siliceous rocks immersed in gabbro frequently show 

 rims of a granitic nature about them, and that fragments of quart- 

 zite immersed in nephelite syenite may show rims of alkaline granite. 

 These and like observations at least demonstrate the possibility of 

 certain reactions, though one may still doubt their quantitative 

 importance in petrogenesis. In the case of the reaction between 

 feldspar-bearing magma and limestone to produce feldspathoid- 

 bearing magma no support of this kind is to be found. Magmas of 

 all subalkaline types have at times included blocks of limestone that 

 have been made over into lime-silicate rocks, yet in no case are 

 these inclusions surrounded by rims of alkaline rock. The 

 limestone-syntectic hypothesis lacks, therefore, even that measure 

 of support that is afforded syntexis in other cases through the occur- 

 rence of the reaction aureoles mentioned. 



SUMMARY. 



In this paper some of the objections that have been raised 

 against the theory of crystallization-differentiation are considered, 

 and its adequacy to explain certain phenomena, for which it has 

 been considered to fail, is pointed out. The supposed advantages of 

 liquid immiscibility in explaining discontinuous variations are 

 considered, and reasons are given for believing that no such advan- 

 tages exist. On the basis of crystallization explanations are sug- 

 gested for discontinuous variations, particularly those noted in the 

 association gabbro-granophyre. A suggestion is made as to the 

 origin of primary banding with particular reference to the Duluth 

 . lopolith. 



