PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 901 



Graywacke.] 



them are striated. The coarser grains are rather evenly distributed in the midst of 

 the fine, but in size they grade downward to the fineness of the matrix. Indeed, 

 there is no way of separating the grains distinctly into coarse and fine, except arbi- 

 trarily, as the coarse grade into the fine. Still the coarser are sometimes prevail- 

 ingly in one part of the slide and wanting in another, the texture varying in its 

 average coarseness. Quartz is more common than in the last. One section. 



Age. Upper Keewatin. N. H. w. 



No. 604G. GRAYWACKE. ( Granitized. ) 



Same place as the last. 



Ref. Annual Report, xx, page 72 ; Annual Report, xxi, pages 37, 51. 



Meg. Undistinguishable from the last. Some close, irregular joints, opened by 

 the fracture, show epidotic lining. 



Mic. This averages somewhat coarser, but is still a fine-grained rock. Some of 

 the feldspars, in the regrowth, show a clear peripheral portion which surrounds the 

 more clouded central area. Hornblende is more plentiful. It is granular and ragged, 

 and but faintly pleochroic. Some of the feldspars are distinctly plagioclastic, with 

 extinction angle between the optic plane and the basal cleavage in a section 

 perpendicular to n s 18, indicating oliyoclase-albite, near albite. The latest to take 

 its present place was quartz. 



This rock has a finely-granitic, interlocking structure, and being wholly crystal- 

 line, might be classed amongst the crystalline rocks approaching granite. One section. 



Age.. Upper Keewatin. N. H. w. 



No. 605G. GRAYWACKE. ( Granitisxd. ) 







Same place as No. 601G to 604G. 



Ref. Annual Report, xx, page 72; Annual Report, xxi, pages 27, 51. 



Meg. Fine grained, gray, similar to the last, but rather coarser. 



Mic. In this it is apparent that the hornblendic masses are finely granular, the 

 short individual hornblendes lying at sixes and sevens with each other. This prob- 

 ably accounts for the absence of evident pleochroism in the hornblendic masses, since 

 the general effect would be monochromatic and greenish in such a composite. 



In this slide are larger examples of a mineral that exists also in the foregoing 

 (from No. 601G), but in so fine a state that its name could not be ascertained. It is 

 apparently garnet. It is highly refractive, but usually dark or faintly light at four 

 points on rotation between crossed nicols. It is water-clear, but has a wide refrac- 

 tive margin on lowering the lower nicol. It is most frequent in the vicinity of the 

 composite hornblendic masses, and is even included within them. It is in irregular, 

 and sometimes sprawling, shapes, without cleavage. 



This rock might be called. a fine-grained granite. One section. 



Age. Upper Keewatin. N. H. w. 



