850 THE GEOLOGY OP MINNESOTA. 



[Porphyrel. Green wacke. 



Throughout the slide is more or less obscure isotropic chloritic substance, the 

 result of alteration of hornblende. This is frequently collected in shreds and patches 

 of larger size, characteristic of fragmental debris. One section. 

 Age, Archean (probably Upper Keewatin). 



Remark. Were it not for the positive field relations it would be difficult to 

 affirm that this is not an eruptive of two dates of consolidation, though much altered. 



N. H. w. 

 No. 2171. PORPHYREL. (Tuff.) 



Same place as the last, in another little ridge a little further south. 

 Ref. Annual Report, xxiv, page 55. 



Meg. This is " porphyritic " with similar feldspars as No. 2170, also with horn- 

 blendes, but at the same time is a pebbly conglomerate. 



Mic. The slide consists of fragments of feldspar and of hornblende, of all sizes, 

 but very few perfect crystals of the former, with much calcite. One section. 



Age. Archean (Keewatin). 



Remark. This is quite similar to several slides already described from the 

 vicinity of Kekequabic lake. The materials show very little effect of abrasion by 

 wave-action, and as there is no other known source for such material, this rock is 

 necessarily classed as of volcanic (tuff) origin. N. H. w. 



No. 2175. GREEN WACKE. (Tuffaceous. ) 



N. W. % N. W. J^ sec. 33, T. 64-9, a little north of Flask creek, near Moose lake ; near the same place as 

 No. 2171. 



Ref. Annual Report, xxiv, page 75. 



Meg. Green, massive-looking, apparently a bed in the prevailing conglomerate, 

 but outwardly having the petrographic aspect of a greenstone. Bed is twenty to 

 twenty-five feet thick. 



Mic. With less calcite than in No. 2171, and with a considerable angular quartz, 

 this rock still is of the same category as No. 2171, etc. It shows little or no beach- 

 action, such as rounding of the grains, but it has a large amount of supposed devit- 

 rified volcanic glass, such glass being now mostly chloritic and isotropic, but showing 

 a few small points in which polarized light passes, as if minute feldspars or quartzes 

 were embraced. In the main, it is composed of fragmental hornblendes, feldspars 

 and quartz, with pebbles which now are composed of micro-granulitic mosaic of 

 feldspar and quartz, but which at first were probably feldspars. Some of the 

 original pebbles were microlitic with feldspar, showing still a fluidal structure in 

 the parallel arrangement of the little feldspars, or a "diabase structure." One 

 section. 



Age. Archean (Keewatin). 



