[brock] contact METAMORPHISM OF A GRANITE ROCK 177 



"The Boundary Creek rock will be found on analysis to contain 

 a greater percentage of alkaline earths but this may be due to the ma- 

 terial it has acquired from the rocks into which it has been introduced 

 and may represent only a local peculiarity. As the Nelson granite 

 occurs to the north and east of this district and probably also to the 

 west, the Boundary Creek rock in all probability belongs to the same 

 great intrusion. If so, its age will be about Jurassic. This agrees 

 with its stratigraphical position in this district." 



The granodiorite, altered to garnet, is found on Pass Creek, in 

 the northeast corner of the Boundary Creek sheet, near a contact of 

 pulaskite, which cuts and dykes it. It is a medium to coarse-grained 

 rock, of pure grey colour, rich in coloured constituents, (dark green 

 hornblende and some biotite) as well as in plagioclase, orthoclase and 

 quartz. Its habit is typically that of a granodiorite. 



Under the microscope, magnetite, titanite and a few zircons are 

 the usual accessory minerals. Orthoclase is partly replaced by mic- 

 rocline. The plagioclase approximates AbsAn, and is therefore chiefly 

 oligoclase. It is frequently altered to kaolin. The characters of the 

 texture and minerals are those common in granodiorite and need no 

 detailed description. 



The normal aspect of this rock is shown in Plate I. Fig. 1. 



This granodiorite is rather heavily dyked by pulaskite and pulas- 

 kite porphyry and is in places altered to garnet, sometimes solid, 

 almost pure, garnet masses, sometimes accompanied by notable quan- 

 tities of other contact minerals. 



The solid garnet masses may be dyke-like, attaining a width of 

 ten feet. The transition from the unaltered granodiorite is usually 

 gradual extending over several feet. The first symptom of alteration 

 from granodiorite to garnet seen in approaching a garnet mass, is a 

 greenish discoloration of the rock, giving it a spotted appearance. A 

 short distance beyond, garnet becomes distinct as small greenish cry- 

 stals scattered through the parent mass. Not infrequently a nucleus 

 of granodiorite is left in the centre of a crystal. Plate I. Fig. 2. 

 illustrates this phase. The dark portions of the photograph are garnet. 

 Proceeding inward the garnet crystals become larger. Their growth 

 is marked by a pronounced zonal structure. As the garnets increase in 

 size, the granodiorite matrix is correspondingly diminished until finally 

 the individuals coalesce and a solid mass of garnet results. When the 

 ' matrix has been reduced to less than half of the mass, it becomes al- 

 tered to a greenish material. 



Plate II, Fig. 3 illustrates a stage considerably more advanced 

 than Plate I, Fig. 2. The garnet individuals are here large; the 

 granodiorite matrix is reduced to less than half the mass and altered 



