524 THE GEOLOGY OF MINNESOTA. 



(Slate. 



Mic. The section is composed of fragments, both of minerals and rocks, of 

 various kinds. The fragments are sharply angular, but more commonly sub-angular, 

 and some are partly rounded. The well rounded grains so common in ordinary 

 sandstones do not occur. Of the minerals quartz and fel<l*p<ir are abundant. The 

 former is the usual granitic quartz and the latter is considerably clouded and 

 kaolinized. Much of the feldspar is plagioclase, which is commonly acid rather than 

 basic. The following minerals are also present: ni/cite, Muscovite, rpii/ote, .fphc.iu', 

 ^oinifi', chlorite and almost colorless, fibrous amphibole. Among the rock fragments 

 may be recognized those of andesyte, apparently devitrified rhyolyte, graywacke, 

 jaspilyte and possibly carbonaceous slate. 



Three sections examined. 



* 



At//'. Ogishke conglomerate at the base of the Upper Keewatin. u. s. G. 



No. 745. SLATE. (Hi/i<-n>ns.) 



N. 10. ',4 sec. l!(i, T. (!.:> (J \V.; near Ogishke Muncie lake. 

 Ref. Annual Report, x, page 92. 



Mi'//. The hand specimen is a slate which shows a number of bands of finer 

 and coarser grained material. The rock is hard and very dark gray in color, but 

 weathers much lighter. Some of the bands contain quartz grains large enough to 

 be seen by the unaided eye. In addition to an imperfect slaty cleavage parallel to 

 the bedding, the rock shows much fracturing and some minute faults. 



Mic. The slide shows a fine banding, due to the concentration of the darker 

 particles of the rock into bands which are not very sharply defined. The rock is 

 made up of a very fine-grained aggregate of quartz, possibly some fe/</*/><ir, dilurilc, 

 opaque substances, calcite and muscovite. A little epidote is present. The opaque 

 substances appear to be magnetite, hematite and other material which is gray in 

 reflected light. A few quartz grains, much larger than the usual grains of the rock, 

 are seen. 



One section examined. 



Age. Keewatin; probably Lower Keewatin. 



Remark. From the field notes it seems quite probable that this rock is part of 

 the Lower Keewatin lying unconformably below the Ogishke conglomerate. The 

 reasons for this inference are the presence of the conglomerate, the abrupt change in 

 strike, and the non-slaty and non-fractured condition of the conglomerate when com- 

 pared with the slate. The field notes are as follows:* 



"There is a sudden change in the direction of the strike of the rock of the 

 country, in N. E. J sec. 26, from nearly east and west to north and south. At the 

 shore on the peninsula, in section 23, it is the same as the slates generally seen in 



*Tenth Annual Report, p. 92. 



