782 THE GEOLOGY OF MINNESOTA. 



[Gneiss. Muscovadyte . 



Me.g. Collected as a doubtful rnuscovadyte. 



Mic. This rock shows all the characters of the finer portions of the Ogishke 

 conglomerate, angular quartzes and feldspars, with shreds of hornblende and occa- 

 sionally of biotite, in a micro-granulitic matrix like that which has been frequently 

 mentioned. This matrix occasionally takes on forms that suggest the former exist- 

 ence of old feldspars, or pebbles of glass. Owing to the comparatively good preser- 

 vation of the evident feldspar grains, and the completely micro-granulitic structure 

 of these pebble-like areas, there seems to be good reason to consider them as 

 originally of glass. Actinolite fibres are scattered throughout the slide. One 

 section. 







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



No. 1777. GNEISS (with biotite.) 



From the long point in the northeast part of Gabemichigama lake, near the centre of W. % sec. 32, T. 

 65-5 W. 



Ref. Annual Report, xxi, page 159. 



Meg. Gneissose. (Compare No. 1350.) 



Mic. This is a more coarsely micro-granulitic condition of the same formation 

 as No. 1776, with a more profound recrystallization. Quartz, biotite, feldspar form a 

 mutually interlocking crystalline rock, penetrated by fine needles of actinolite. There 

 are evident remnants of the old feldspars. These areas are less coarsely micro- 

 granulitic, and they are also distinguished by fine brightly-polarizing scales that 

 resemble sericite. The secondary feldspar gives an extinction angle on n p of 64, 

 and n, is in the acute optic angle, indicating andesine. In this rock the biotite was, 

 in general, earlier than the quartz, but there are noticeable exceptions. One 

 section. 



Age. Archean (Upper Keewatin). 



Remark. It appears from this and other slides that, petrographically, the 

 Ogishke conglomerate grades into the gneiss of the northeastern part of Gabemichi- 

 gama lake, into the "greenstone" of the region, and into the green schists of 

 Kekequabic lake, as well as into the muscovadyte at Disappointment lake and the 

 porphyrel of Zeta lake. N. H. w. 



No. 1778. MUSCOVADYTE. 



N. E. J N. E. J sec. 34, T. 65-5 W., near the junction of two branches of the creek. From the top of the 

 cliff facing north. 



Ref. Annual Report, xxi, page 159. 



Meg. Fine grained, gabbro-like. 



Mic. The elements are all fine, and consist of some plagioclase, which was 

 about cotemporary with the pyroxene, of magnetite and of biotite. One (thick) section. 

 Age. Cabotian (recrystallized Keewatin). N. H. w. 



