PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 343 



Quartz-Porphyry.] 



feldspar forms by a darkening which spreads over them four times in each revolution 

 between crossed nicols. They are not perfect, i. e., idiomorphic, but are fragments 

 of feldspars, and they appear to be mingled with many small fragments of the same 

 kind. The groundmass, made up of more minute particles of feldspar and of quartz, 

 is mainly homogeneous, as a groundmass, but sometimes it shows variations in the 

 sizes and relative proportions of the feldspathic and quartzose elements, so that 

 darker areas appear in the midst of the general mass. Such darker or coarser areas 

 are themselves apparently of fragmental origin, i. e., it appears that fragments of 

 rock, though mainly like the average, yet varying somewhat from it in special 

 characters, were included in this rock. What may be the cause of these variations 

 it is not possible as yet to state. They are frequently considered the result of 

 crushing and shearing of a homogeneous quartz-porphyry, or of a siliceous tuff, or 

 porodyte, whose fragmental elements originally differed slightly in composition, as 

 well as in structure, but that explanation seems insufficient. 



Two sections. 



Age. Archean (Keewatin). 



Remark. The determination of the true nature of this rock has an important 

 bearing on the Stuntz island conglomerate, since it is apparently in the line of 

 extension from that place. So far as the samples now at hand in thin section will 

 show this nature, it is impossible to say positively whether this rock was originally 

 an acid igneous one, i. e., a quartz-porphyry, or an acid tuff, i. e., porodyte, but the 

 question will remain open till other sections can be examined. 



Later examination has shown that Stuntz island is composed of two sorts of 

 rock, one a quartz-porphyry, a massive crystalline rock, occupying the northern part 

 of the island, and the other a coarse fragmental largely derived from the other, 

 making the well-known Stuntz conglomerate. Rock No. 383 may be considered a 

 finer and more siliceous phase of the conglomerate. In the field this was called 

 porodyte, with the idea that it may have been an acid tuff. N. H. w. 



No. 384. QUARTZ-PORPHYRY. (Altered. ) 



Vermilion lake; probably in N. W. % S. W. X sec. 13, T. 62-15 W. 

 Ref. Annual Report, ix, page 97. 



Meg. A roughly schistose, soft, greenish-gray, very fine-grained rock. Contains 

 a few large quartz grains, altered feldspars, and pyrite. 



Mic. The section shows a much sheared and decayed quartz-porphyry. The 

 feldspars are broken, much decayed and are in places hardly distinguishable from 

 the groundmass. The rock as a whole is very similar to No. 375. One large quartz, 

 which is partially idiomorphic, shows embayments of the groundmass similar to those 

 seen in unaltered quartz-porphyries. On one side of a large pi/rite crystal is a greenish 



