320 THE GEOLOGY OP MINNESOTA. 



[Schist. Granite. 



grayish to greenish, or even clear and translucent in natural light. In some cases a 

 fibrous or scaly structure like chlorite is visible, but in most of the grains of this 

 sort, the clear parts are simply the background for many microliths, some of which 

 are opaque and some transparent and capable of polarizing light. 



The slide also contains areas of calcite, and one cubic section of pyrite, in the 

 vicinity of which is some chalcedony, at least a minutely fibrous glass-clear mineral 

 in which the elongation is sometimes negative and sometimes positive. 



The slide also shows variations in the relative abundance of the opaque grains. 

 These are sometimes so grouped as to suggest that fragments of some different rocks 

 were involved in the accumulation of this. These fragments are not vesicular, dis- 

 tinctly, but seem to be composed of rock similar to the rock in which they lie. 



One section. 



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



No. 329. SCHIST, f Horriblendic. ) 



East end of Bassimenan (Basswood) lake, near Prairie portage; probably in N. E. ^4 S. E. J^ sec. 2, T. 64-9 W. 

 Ref. Annual Report, ix, page 87. 



Meg. A rather soft, dark-green rock, slightly schistose. The rock is too fine- 

 grained to allow the components to be distinguished. Along seams some calcite is 

 deposited. 



Mic. The section shows green hornblende, calcite, quartz, plagioclase, epidote, 

 chlorite and pyrite. The hornblende, which is the most abundant mineral, is elon- 

 gated in one common direction, thus giving a decided schistose structure to the section 

 much more pronounced than would be thought from an examination of the hand 

 sample. The hornblende is wrapped around quartz and plagioclase grains which show 

 no clastic features, but are fresh and interlock with each other. Scattered through 

 the section is abundant calcite (probably in part siderite) and small flakes of chlorite. 



One section. 



Age. Archean (Keewatin). 



Remarks. This rock is part of the "greenstones" of the Keewatin. From the 

 field notes, the hand sample and the section it is impossible to draw any definite con- 

 clusion as to the original nature of the rock. It shows no indication of structures 

 which are definitely characteristic of either clastic or igneous rocks. It seems 

 probable that the rock represents a sheared and recrystallized diabase or basic ash. 



U. S. G. 



No. 330. GRANITE (with Iwrriblende, ). 



Bassimenan (Basswood) lake, N. E. J^ N. W. J see. 2, T. 64-9 W. 

 Ref. Annual Report, ix, page 87. 



Meg. Rock varies from rather dark to lighter colored, as the hornblende and 

 micaceous elements vary. 



