41 



been successively and more or less independently injected into 

 their present positions from an underlying magma reservoir. 

 The intrusion is clearly transgressive, and it is difficult 

 to know what term should be applied to it. It is not a true 

 laccolite, inasmuch as it is not concordant; the term 

 "chonolite" proposed by Daly, though rather comprehensive 

 in its defined significance to be of much practical value, comes 

 nearest to describing the form of the intrusion. 



(5) MECHANICS OF DIFFERENTIATION. 



To establish the sequence of events by which the 

 assemblage of rocks was produced from the original magma 

 is by no means easy, in view of the paucity of exposures and 

 the possibility that some of the members of the series do not 

 outcrop or have never been erupted from the intratelluric 

 reservoir. 



In the author's opinion the facts of intrusion, as we see 

 them, can best be explained as due to two different types of 

 differentiation : (1) fractionation with sinking of crystals, 

 and (2) complementary differentiation, or the splitting of a 

 portion of the original magma into an acid and a basic frac- 

 tion. Differentiation in situ has also operated, but to a very 

 minor extent. 



(1) The possibility of some degree of differentiation by 

 the sinking of crystals in a fluid magma has long been recog- 

 nized by geologists, and the idea has been elaborated by 

 Bowen,(^^) who would assign to it a first place in the production 

 of rock species. There are certain facts in connection with 

 the Encounter Bay rocks which point to subsidence differenti- 

 ation as a factor in their evolution. Attention has already 

 been drawn to the linear relationship between the specific 

 gravities and the variation in iron, magnesia, and soda which 

 exists in the case of the quartz mica diorite and the two 

 granites. Other facts which seem to be significant are: — 



(1) The decrease in specific gravity from diorite to 



even-grained granite. 



(2) The progressive decrease in biotite and increase in 



the felspars and quartz. 



(3) The progressive acidity of the plagioclase. 



(4) The very fine grain of the quartz mica diorite, 



increasing in the porphyritic varieties, and the 

 increase in size and abundance of phenocrysts 

 corresponding with the increasing grainsize of 

 the ground-mass. 



(33) Supplement to Journ. Geol., vol. 33, 1915. 



