LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 53 



these the control of gravity may be reasonably inferred, though 

 the heavier differentiates may occur at unobservable depth in the 

 case of the larger intrusions. In the following a few such bodies 

 are described and in some cases the facts on which the foregoing 

 inference may be based are stated. 



The Hope bathoHth, a late portion of the Coast Range batho- 

 lithic complex sectioned by the Fraser River Valley, illustrates 

 differentiation on a huge scale, as does its southward continua- 

 tion the Chilliwack bathoHth.^ The types represented vary con- 

 tinuously from gabbro-diorite to granite, and the mineralogy of the 

 individual t3rpes is instructive in this connection. The gabbro- 

 diorite is a labrodorite-hypersthene rock. As the plagioclase 

 becomes more sodic, hornblende replaces hypersthene, at first 

 partially and finally completely. Biotite then begins to take the 

 place of hornblende, but only in those phases which have also free 

 quartz, and as these two appear, orthoclase also becomes important. 

 Finally when quartz and orthoclase are fairly abundant, biotite 

 becomes the dominant colored constituent.^ 



Precisely the same type of differentiation is shown in the 

 plutonic complex in the vicinity of Hedley, the variation being 

 from gabbro to biotite granodiorite. The relations at this place are 

 such as to indicate that assimilation had no part in the formation of 

 the saHc types. The small and quickly cooled sateUitic stocks and 

 sills of gabbro and the diorite-gabbro phase occurring as a chilled 

 border, representing restricted differentiation, about the somewhat 

 larger quartz diorite stocks seem to show that the original magma 

 when it arrived at its present level was still normal gabbroic magma 

 and had not been acidified. Crystallization and the settHng of 

 crystals from this magma gave rise to the diorite in the stocks 

 and, in a more advanced form, to the granodiorite of the batho- 

 lithic mass. 



The dioritic intrusives of the Prince of Wales Islands, Alaska, 

 satellitic to the Coast Range bathohth, show this same tendency 



' R. A. Daly, "Geology of the North American Cordillera at the Forty-ninth 

 Parallel," Geol. Survey Canada, Mem. 38, p. 534. 



^ N. L. Bowen, Guide Book No. 8, Part II, XII""' Congres Geologique Interna- 

 tional, 1913, p. 258, mentioned there under the field designation "hornblende-rich 

 granitic rocks" on account of the dominance of the intermediate types. 



