156 A. Wand he— Intrusive Rocks of 



Differentiation. 



The stocks and batholiths of the Portsmouth Basin, 

 although not remarkable for an assemblage of rare rock 

 types, nevertheless exhibit contrasted mineralogical com- 

 positions. In the descriptions of the igneous bodies it 

 was mentioned that the composition of the Durham 

 quartz diorite varied from a basic marginal phase to an 

 acidic central one ; that the Agamenticus complex seems 

 to show a march of progress from calcic to alkalic types ; 

 that the dikes in their cross-cutting, relationships indicate 

 a change in composition of the parent magma from a 

 basic to an acidic type. The foregoing statements appear 

 to be the field facts ; their bearing upon the problem of 

 rock origin seems worthy of consideration. 



Of the two contrasting types of rock — the calcic and 

 the alkalic — the former appear to have been developed 

 by the simpler processes. The quartz-rich calcic con- 

 tact phases of the Durham quartz diorite seem quite 

 clearly the result of the assimilation of Kittery quartz- 

 ite by the gabbroid magma. The study of a number 

 of selected specimens in which fragments of a quartzite 

 were seen in various stages of alteration seems to prove 

 this point. The abundance of apatite, indicating the 

 presence of plenty of mineralizer, would, moreover, 

 appear to show that the assimilation was aided by the 

 action of the rock's volatile components. But the grada- 

 tion from a gabbroid margin to a granitic center seems 

 hardly the result of assimilation as no granite was seen 

 in the marginal phase. The formation of granite might 

 in part result from a process of fractional crystallization 

 under gravitative control as the laboratory work of 

 Bowen 18 indicates may take place experimentally. But 

 such a simple setting is hardly the entire story. In 

 studying by means of thin sections several suites of 

 specimens taken across the Durham body, two facts 

 stand out rather clearly: (1) the gradual disappearance 

 of pyroxene and the development of hornblende and 

 biotite, both minerals that all experimental work indi- 

 cates can only be formed in the wet way; (2) the partial 

 disappearance of calcic feldspar and the development of 

 sodic and potassic feldspar, the latter again, judging 



18 Jour. Geology, vol. 23, No. 8, suppl., 1915. 



