THE KEEWATIN AND THE ARCHEAN 509 



in mineral composition the resulting metamorphic rocks vary greatly. 

 A very prominent feature of these altered rocks is the presence of much 

 hiotite, while in the intermediate vicinity of the contact h}-persthene is 

 developed frequently in poikilitic plates. 



THE ARCHEAN 



Along the northern border of the gabbro the Archean is represented 

 by two types of rocks, granites and greenstones. Contact metamorphism 

 in the first of these has not been noted ; in fact, it may be questioned 

 whether the gabbro induced any marked changes in these acid rocks ; 

 but in the other type, the greenstones, a series of changes has taken place 

 which has produced rocks of a peculiar nature. Originally the green- 

 stones here considered consisted of gabbros, diabases, diorites, or the 

 finer grained extrusive and fragmental equivalents of these rocks. They 

 have been subjected to many vicissitudes, and in their present composi- 

 tion they are aggregates of hornblende, plagioclase, kaolinite, epidote, 

 quartz, and minor alteration products. The metamorphism induced by 

 the gabbro has been of a nature which has tended to reproduce the 

 original minerals of these greenstones. The result is in some cases quite 

 similar to a fine grained gabbro. In fact, some of these metamorphosed 

 greenstones have been described as parts of the great gabbro mass, while, 

 on the other hand, certain fine grained and granulitic phases of the 

 gabbro have been referred to these changed greenstones. The resemblance 

 between the two rocks is quite marked on surfaces which have been ex- 

 posed to the weather, both forming yellow granular masses, which 

 crumble readily under the hammer. There is, however, a difference 

 between these apparently similar rocks of diverse origin, and a judicial 

 combination of field evidence and microscopical study will commonly 

 enable one to decide to which category the rock of a given outcrop be- 

 longs. The granulitic gabbros are confined within (that is, to the south 

 of) the northern boundary line of the gabbro mass. This line can be, 

 and in fact for a considerable distance has been, carefully located. The 

 metamorphosed greenstones are without (that is, to the north of) this 

 boundary, and, moreover, they can usually be traced directly into the 

 less altered greenstones, the zone in which the complete recrystallization 

 has taken place being commonly a comparatively narrow one. In thin 

 sections the granulitic gabbros present a typically granular texture and 

 the grains are very uniform in size. Hornblende is not common, and 

 olivine and hypersthene are at times found. 



In the metamorphosed greenstones the texture is not so typically 

 granular, and the grains vary considerably in size, although many of 

 them have the approximately circular outlines which are so noticeable 



