72 The Ottawa Naturalist. [June 



These facts bearing- so intimately on the genesis of ig-neous 

 rocks have caused the formulation of the hypothesis known as 

 magmatic differentiation. This hypothesis or phenomenon may be 

 briefly described as the division or differentiation of a more or less 

 viscous or molten magma or fused mass of rock, into chemically 

 and mineralogically diverse parts, which on cooling, yield corres- 

 pondingly diverse types of rock. It would be manifestly unwise 

 in this connection to enter into a discussion of this very generally 

 accepted hypothesis, as the conditions attending the crystallization 

 or consolidation of a large body of mag-ma are now believed to be 

 much more complex than at first supposed. Moreover, our 

 knowledge regarding these conditions and the several processes 

 which are no doubt involved, is so vague and incomplete, that no 

 full or satisfactory explanation has yet been offered of this 

 phenomenon. All petrologists of repute are, however, agreed on 

 the main fact that magma differentiation furnishes the only reason- 

 able explanation of the facts observable in regard to the mode of 

 occurrence or field relations of these rocks. In the present instance 

 it will doubtless be sufficient to state that mineralogically the 

 exposures at Ice river furnish abundant proof of their intimate 

 relation to one another, and their derivation from the same magma. 

 This strong disposition towards differentiation is a marked feature 

 of nepheline syenite and kindred eruptives, and it is very reason- 

 able to suppose from the studies already undertaken that nowhere 

 in the world is it better exemplified than by these British Columbia 

 outcrops. 



The following rather distinct types of rock have been recog- 

 nized in the hand specimens collected by Dr. Dawson, which with 

 the intermediate or transitional forms, exhibit a rather perfect 

 gradation throughout the series. 



I. A rock composed mainly of hornblende, pyroxene and 

 nepheline, together with sphene, which although accessory, is so 

 abundant as almost to characterize the rock. Occasional thin 

 sections show small individuals of anorthoclase, but as a rule no 

 felspar is present (Ijolite). 



Macroscopically the hand specimen representative of this type 

 is a black glistening rock made up chiefly of the darker coloured 

 minerals (hornblende and pyroxene) with a very subordinate 



