﻿484 BR. WHITMAN CEOSS OX THE NATURAL [Allg. I9IO, 



To me the occurrence of the rocks described in 

 Colorado seems incompatible with the idea of their 

 exclusive association with foyaitic-theralitic magmas, 

 although I grant that experience shows them to be elsewhere 

 frequently connected genetically with such magmas. 



The general relations of camptonite occurrences. — 

 Examining the statements of Eosenbusch concerning the distribu- 

 tion of camptonite, in his w Physiographic,' we find the frank avowal 

 that the derivation of the original type camptonites of New England 

 from foyaitic magmas was inferred by him from their frequent 

 association with bostonitc. But not all of these rocks occur in 

 close connexion with foyaitic-theralitic magmas. Eocks of the 

 granito-dioritic series also occur in the region. The original 

 camptonites described by Hawes are many miles distant from the 

 nearest occurrence of alkalic rocks at Eed Hill, and no alkalic 

 parent magmas are known in connexion with them. The numerous 

 dykes of camptonite described by Kemp and Marsters are much 

 nearer to the great anorthosite and gabbro masses of the Adiron- 

 dacks than to any foyaitic bodies. Camptonite and bostonite *or 

 keratophyre also occur in the Adirondacks. 



Eosenbusch asserts that the lamprophyric dyke-rocks of the one 

 great series are never connected with those of the other by transi- 

 tional forms, and in accordance with this belief assigns all rocks 

 described to a definite place. It is manifestly impossible to deter- 

 mine in the field the genetic association of many camptonites, 

 bostonites, kersantites, and aplites. From the vast amount of 

 evidence that the two magmatic series are connected by equally 

 abundant intermediate magmas, T can but believe that the sharp 

 distinction between the two 'dyke rock' groups is a 

 purely arbitrary one, resting on an unproved hypo- 

 thesis. 



Conclusion as to 'dyke rocks.' — Our present knowledge 

 of the processes of magmatic differentiation is too slight to permit 

 a definite assertion, that from a given parent magma certain 

 differentiates must be formed. But it is reasonable, I think, to 

 suppose that typical foyaitic, monzonitic, and dioritic magmas, 

 subjected to the same conditions, must produce differing series of 

 descendant partial magmas. It does not necessarily follow that 

 those series must be markedly different in all their members. If 

 different conditions attend the differentiation of the magmas, the 

 resultant partial magmas may not fully correspond to those formed 

 under uniform conditions. 



Given an intermediate monzonitic magma, is it not natural to 

 suppose that its descendant magmas must be intermediate in many 

 respects between the series derived from foyaitic and dioritic parent 

 magmas, and that a shifting of conditions may throw the dominance 

 of characters either one way or the other? 



