PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 855 



Granite-porphyry. Tuff.] 



old crystal had suffered a partial mechanical as well as chemical disintegration. 

 The surrounding plexus, so far as it is of new development, consists of interlocking 

 small feldspars and hornblende in which the hornblendes are earlier, and of quartz 

 and epidote. Throughout this fine mass are plainly smaller "old feldspar" grains or 

 fragments not entirely replaced by the secondary growth. These contain the same 

 inclusions and are supplied with the same surrounding fringe as the large crystals, 

 and appear to be simply such original grains as have not been entirely lost in the 

 general transformation, but the groundmass in general is wholly new and fresh. 

 The older ingredients, not of feldspar, which are found in this interlocking matrix, 

 are sphene, apatite, magnetite (perhaps of later date) , garnet. These are in insignificant 

 amounts. The hornblendes are small and are arranged in a gneissic structure. One 

 section. 



Age. Archean (earlier granite). 



Remark. This rock resembles the so-called porphyry seen at various places, 

 and it is evidently an older intrusive than the granite which cuts it. It has the 

 structure and appearance of the porphyroidal granite of Kekequabic lake, but 

 that does not prove equivalence of age. Given a certain (conglomeratic) formation, 

 such as that about Moose lake or about Kekequabic lake, and it is evident that a 

 metamorphism, and even a plasticity, could be produced at different epochs, and 

 that the resultant rock, under similar or identical agents, would be about the same 

 at different places. 



This rock is considered to be essentially the same in original character and 

 genesis as No. 2184, only differing in having more advanced metamorphism. Struc- 

 turally it is essentially an igneous rock, having been forced, when plastic, amongst 

 the adjoining strata, under great pressure. N. H. w. 



No. 2190. TUFF. 



S. E. 54 N. E. 14 sec. 24, T. 64-9, Snowbank lake, near the head of the little bay, but on the north side of it. 

 Ref. Annual Report, xxiv, page 58. 



Meg. Schistose, hornblendic, having dim porphyritic feldspars. This is a part 

 of the great conglomerate of the region, but of indefinite outward characters. 



Mic. There has evidently been a shearing pressure applied to this rock, as 

 indicated by the elongation. This elongation is expressed chiefly by the streaks and 

 bands of epidote and of hornblende, and by a uniform direction of the major axes of 

 the feldspars. The old feldspars are coarsely and much twinned, and are rather 

 uniformly decayed, after the micro-granulitic crystallization. It is also plain in this 

 rock that the hornblende is derived from an alteration from augite, for some old 

 augite forms remain. One section. 



Age. Archean (Keewatin). N. H. w. 



