658 THE GEOLOGY OP MINNESOTA. 



[Biotite gneiss. 



albite twinning, but in a favorable section showing w,, perpendicular the extinction 

 on the twinning trace was 70. This is not conclusive, "but by making the Becke 

 test for comparative refraction it is found that these earlier glassy grains are 

 uniformly less refractive than the quartz ; many of them are certainly of cordierite. 

 Compare Nos. 1350 and 1351. One section. 



Age. Archean (Keewatin). 



Remark. In another section of this rock the recrystallization is more advanced, 

 the grains are larger and interlocking more like the structure of a crystalline massif. 

 But as the rocks of this point are known to be elastics affected by the gabbro, this 

 furnishes an instructive instance of the mannerof formation of gneiss from clastic rode. 



The rocks of this point received detailed examination in the field, and are 

 referred to again under Nos. 1350 and 1351. See also rocks Nos. 767, 768, 983. 



N. H. W. 



No. 1091. BIOTITE GNEISS. 



Same locality. Northwest portion of the S. W. >4 sec. 32, T. G5-5, Gabemichigama lake. 

 Ref. Annual Report, xv, pages 380, 397; Annual Report, xxi, page 148. 



The same kind of rock, but without apparent bedding or gneissic structure. 

 Mic. The same interlocking fine secondary quartz and feldspar as ground- 

 mass, is thickly and uniformly sprinkled with biotite. N. H. w. 



No. 1092. BIOTITE GNEISS (icith cordierite). 



Near the same place as the last, northwest corner S. W. % sec. 32, T. G5-5. 

 Ref. Annual Report, xv, pages 380, 397. 



Meg. Stratified gneissic rock. Compare No. 1350. 



Mic. The rock, consisting entirely of secondary products, is still entirely 

 crystalline. The most abundant element is perhaps the cordierite. This conliri-H<' 

 sometimes wholly surrounds small pyroxene grains which are nearly round. 

 Quartz is less common, but still there are some large grains, and these are spread 

 amongst the other minerals so as to involve them poikilitically, /. e., the small 

 cordierites, the pyroxenes, the biotite, the last also embracing the small pyroxenes. 



The pyroxenes present a singular feature ; they embrace sometimes a number 

 of smaller globular pyroxenes, of differing orientation, and when these are numerous 

 the general extinction is imperfect, and the whole grain appears to be made up of a 

 grouping of smaller individuals. The consequence is that there is no complete 

 extinction, and at the same time the cleavages are not evident. This phenomenon 

 has to be explained in the same manner as quartz-globulaire has been explained by 

 Fouque, and similarly to the siderite-globulaire already noted in this work, viz.: that 

 these infant globules represent the commencement of aggregation into crystals, but 

 never acquired definite crystal outlines. One section. 



Age. Archean (Keewatin). N. H. w. 



