PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 865 



Greenstone. Graywacke.] 



spars are mainly plagioclase there is occasionally one which has a pronounced 

 microcline structure. In such microclines are numerous isolated fresh feldspar grains 

 like the last mentioned, as well as dull areas filled apparently with kaolin, suggesting 

 that the whole microcline grain is due to a regrowth on and in an older feldspar crystal. 



Throughout the slide are also shreds of a highly refractive and doubly refractive 

 mineral which resembles epidote, but the grains are so small and their color in 

 common ligh'u is so white that it is not possible to affirm that these grains are not a 

 light-colored augite. There is also a little sphene and a very small amount of green 

 hornblende. One section. 



Age. Lower Keewatin. N. H. w. 



No. 2231. GREENSTONE. 



Phase or part of No. 2230 ; same place as No. 2229. No. 2230 contains rounded and angular pieces of No. 

 2229 and much finer debris, constituting a conglomerate. 

 Ref. Annual Report, xxiv, page 69. 



Meg, Firm, fresh, uniform, massive aspect in the field, with much hornblende 

 or pyroxene. 



Mic. Hornblende constitutes the coarsest and most conspicuous part of the 

 slide. It is nearly colorless and almost without pleochroism. Its angle g :c=16. 

 It has positive elongation in a section showing n f perpendicular, and its acute 

 bisectrix is n v . If this be an aluminous hornblende it is edenite*, but optically it is 

 also much like tremolite. 



Epidote, which is also almost colorless, is widely disseminated through the section, 

 and frequently is embraced in minute grains in the foregoing amphibole. It consti- 

 tutes in minute granules the major portion of some areas that were formerly either 

 feldspar or some ferro-magnesian mineral, probably the former, because remnants 

 of an old feldspar are sometimes seen in the midst of such an epidotic mass. 



The feldspar is so altered that it cannot be specifically determined. It consti- 

 tutes but a minor portion of the larger grains. It but rarely shows any albite 

 or other twinning. One section. 



Age, Upper(?) Keewatin. 



Remark. Owing to the great variability in the amount of decay of the feldspars, 

 the manner of connection of the grains into a rocky structure, and the great preva- 

 lence of finely granular epidote, it is perhaps reasonable to assign this rock to a 

 clastic origin. Its close connection with rock No. 2230, which in the field was 

 not detected to show an igneous contact, favors that view. N. H. w. 



No. 2232. GRAYWACKE. 



At a quarter of a mile north of the east quarter-post of sec. 8, T. G3-8. 

 Ref. Annual Report, xxiv, page 09. 



* Minh-ftlogic iff la Franre, ii, pp. 634, 

 56 



