860 THE GEOLOGY OP MINNESOTA. 



[Granite. Muscovadyte. 



Mic. In a finely granulitic groundmass are zoned and twinned crystals of feld- 

 spar, and imperfectly formed crystals and shreds of hornblende, with some biotite. 

 The appearance is like the esterellyte of Kekequabic lake, with the exception that 

 the ferromagnesian element is less abundant, and does not approach augite. The 

 central portion of the feldspars is frequently converted to an obscure, crowded mass 

 of minute crystals of mica, epidote and secondary feldspar. That the groundmass 

 was at first fragmental, and that the zoning of the feldspars was coincident with the 

 micro-granulitic development by which most of the smaller fragments have lost their 

 identity, is an obvious conclusion which is in keeping not only with the present con- 

 dition of the feldspar crystals, but also with the heterogeneous aspect of the micro- 

 granulitic groundmass itself. The principal zone of the feldspars has an extinction 

 on n f of about 3, indicating andesine-oligoclase, but the exterior zone has an extinc- 

 tion angle, in the same crystal, of about 17, which is near that of albite, while the 

 kernel is so obscured by inclusions of mica, etc., that its exact extinction can hardly 

 be determined. Several trials, however, on the kernel, which has not a uniform 

 simultaneous extinction, but a fragmentary or wavy one, give angles varying from 

 13 to 18. Not much reliance can be put on this indication, but, if it be supposed 

 that the variation in acidity was in the same direction at the centre as at the 

 periphery (as is'usually the case), these figures point toward labradorite as the original 

 feldspar. One section. 



Age. Archean (dike). 



Remark. It is reasonable to infer that this granite is analogous to that cutting 

 the green schists at Kekequabic lake. N. H. w. 



No. 2208. MUSCOVADYTE. ( Micaceous. J 



S. W. y N. E. J^ BCC. 5, T. 63-8, Disappointment lake, at the lake shore. 

 Ref. Annual Report, xxiv, page 64. 



Meg. Apparently a mica schist, but an approach toward muscovadyte. This is 

 also plainly a part of the green, pebbly formation which prevails, irrespective of the 

 intrusives, from Moose lake to Disappointment lake. 



Mic. The rock is micro-gran ulitic, with variations in size, some of it being 

 plainly due to replacement of original clastic feldspars, some of which are still pre- 

 served. Hornblende in small round and obtuse grains is the most abundant of the 

 dark minerals, but shares with biotite and magnetite. The old feldspars, so far as 

 they remain, are zoned, and the fringe is interlocked with the surrounding fine 

 groundmass. One section. 



Age. Archean (Keewatin). 



Remark. This is in the line of strike from Cheadle's cabin, but further removed 

 from the gabbro intrusion. N. H. w. 



