PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 891 



Graywacke. Quartz-porphyry.] 



simultaneously. At the extinction point certain bands are faintly darker than the 

 others, and at full illumination certain ones are somewhat brighter than the others. 

 Others preserve the albite twinning distinctly. It is observable also that through- 

 out the finer matrix the secondary feldspar is more freely developed, interlocking 

 with the secondary quartz, some of the fine older grains being surrounded by a new 

 rim, while the large feldspars exhibit usually no rim of new growth, although they 

 are probably largely regenerated throughout, and it appears reasonable to attribute 

 to such regeneration the impairment of the albite alternate maclation. The quartz 

 is wholly remodeled, thus making a firm interlocked granitic structure. One section. 



Age. Keewatin. 



Remark. The modifications of this rock, due to metamorphism (Twentieth 

 Annual Report, page 46), are in nature and structure like those seen in various 

 crystalline rocks which, called granite or dioryte, are intrusive on the adjacent 

 elastics. N. H. w. 



No. 386G. GRAYWACKE. ('Metamorphosed. ) 



Same place as the last. 



Ref. Annual Report, xx, pages 46, 99. 



Meg. Weathers nearly white, apparently consisting of quartz and feldspar, 

 fine grained. 



Mic. The renewed mineral condition of the whole rock is the same as in the 

 last four described rocks, but this rock contains almost nothing but quartz and the 

 white silicates, with epidote which also is itself nearly white. In some instances 

 (noted also in No. 385G), isolated small globular quartz granules are in the feldspars. 

 This seems to imply that the feldspar has undergone a general molecular rearrange- 

 ment. One section. 



Aye. Keewatin. N. H. w. 



NO. 403G. QUARTZ-PORPHYKY. 



Prom a dike in the greenstone north of the Kawishiwi river; N. N. W. ',4 sec. 28, T. (v5-10 W. 

 Ref. Annual Report, xx, pages 48, 99. 



Meg. This rock has a grayish ground mass in which are embedded a few quartz 

 grains and numerous flesh-colored and blood-red feldspars. The quartz is more 

 plentiful near the edge of the dike (No. 402G). In places the rock contains minute 

 cavities, apparently formed by the weathering out of certain constituents of the rock. 



Mic. Under the microscope the rock is seen to have a microgranitic ground- 

 mass of rather variable grain. In this are feldspars of all sizes up to pieces over a 

 quarter of an inch in length. The crystal outlines of the feldspars are not usually 

 distinct and most of them show no planes at all, being pieces with irregular outlines. 

 Many of the feldspars show polysynthetic twinning lamellae, and a large number do 



