680 THE GEOLOGY OF MINNESOTA. 



[Slate. Argillyte. Graywurke. Mnwovadyte. 



Mfg. Porphyritic aspect, with a little pyrite. Compare No. 311. 



Mic. This rock is quite like No. 1279, bat coarser, and the full description need 

 not be repeated. The slide, however, shows no large quartzes, and the mica scales 

 are more abundant. One section. 



Aye. Archean (Keewatin). 



Kemtn-k. On the weathered surface the white porphyritic feldspars stand out 

 prominently and are larger and more numerous than the quartzes. N. H. w. 



No. 12H4. SLATE. (Mi<-ni'ci)u,s. ) 



Interbedded with No. 1283; north shore of Gunflint lake (Canada). 

 Ref. Annual Report, xvi, pages 68, 120. 



Mi if. Fine, grayish, slaty. 



Mir. Mica scales are distributed parallel with the slatiness. Hence the slide 

 darkens noticeably four times in one revolution, like a zeolitic mineral. It contains, 

 however, nilcilc and angular grains of feldspar, which do not usually darken parallel 

 with the threads and the slatiness. N. H. \v. 



No. 1285. ARGILLYTE. 



Interbedded with No. 1283. 



Ref. Annual Report, xvi, pages 68, 120. 



Meg. No specimen found. No section. 



Age. Archean (Keewatin). IT. s. G. 



No. 1286. GRAYWACKE. (Sheared.) 



Same rock as No. 1282 (Canada). 

 Ref. Annual Report, xvi, page 120. 



Meg. Evident quartzes are visible in this rock, which is schistose. 



Mic. While this is much like No. 1283, it differs from it in having much more 

 general decay and less evidently a gneissic structure. Quartz is quite abundant in 

 rounded and sub-angular grains. One section. 



Age. Archean (Keewatin). 



Remark. This is quite evidently a clastic rock, and its relations with No. 1283, 

 as described in the Held, so closely ally it with that rock that it is reasonable to call 

 attention to the petrographic alliance of Nos. 1279 and 1283 with the pebbly and 

 "porphyritic" portion of the Ogishke conglomerate seen intermediate between 

 Ogishke, Muncie and Kekequabic lakes, in which the "porphyritic" feldspars are 

 evidently derived as detritus from some earlier quartz-porphyry. 



No. 1287. MUSCOVADYTE. 



Mayhew iron location, Mayhew lake. Compare Nos. 694-700. 

 Ref. Annual Report, xvi, pages 78, 120. 



Meg. The gray rock "on which the gabbro lies unconformably." 



