ANORTirOSITES OF THE RAINY LAKE REGION 91 1 



Vermilion Lake as representing the truncated base of a Keewatin 

 volcano which served as one of the vents for the pyroclastic 

 materials so widely found in the Keewatin rocks of the region, 

 the basic rock coming first and the acid afterwards/ In this 

 he is probably not correct, for there is good evidence to show 

 that the anorthosite, which probably solidified under a consider- 

 able thickness of superincumbent rock, was so far exposed by 

 denudation that fragments of it could be rolled into bowlders 

 and become part of a conglomerate before the eruption of the 

 granite. The latter rock has sent apophyses into the anorthosite 

 and has pushed its way through a schist conglomerate contain- 

 ing pebbles and bowlders of quartz-porphyry, sandstone, green 

 schist and occasionally also anorthosite quite like some facies of 

 the adjoining mass. Apparently a long interval separated the 

 anorthosite eruption from that of the granite. The sharp segre- 

 gation of a magma into a basic anorthosite and a very acid 

 granite would in any case be rather surprising. 



A. P. Coleman, 

 School of Science, 

 Toronto. 



' Geol. Sur. Can., loc. cit. 



