Abstract

A large stock of alkaline rock is described quantitatively, and the mineral variations are shown on contoured distribution plans. Chemical analyses of albite, microline, and nepheline are presented, as well as chemical analyses of six of the type rocks. Field observations of the relationship to surrounding rocks are supplemented by age determinations based upon radioactivity. The latter indicate an age difference of the order of 500 million years between the feldspathoid rock and the younger granitic batholith, thus precluding the possibility of a genetic relationship between them. Based upon this and upon the lack of evidence of desilication of the intrusives by limestone, it is concluded that the limestone syntexis theory of origin of feldspathoid rocks is not tenable for the intrusive in question and seems unlikely for the Bancroft alkaline province as a whole.

Analogy is drawn between the crystallization of the alkaline intrusive and that of a comparable artificial melt; it is concluded that the characteristics of the intrusive and the mineral variations can be reasonably attributed to crystallization of a magma approximately of the composition of the average rock. There is no evidence that the alkaline magma was developed by any special process operative in the zone of present observation.

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