310 THE GEOLOGY OP MINNESOTA. 



[Slate. Graywacke. 



Apatite is also present, as is also magnetite, both secondary and original. In structure 

 the rock approaches granitic; the augite, while in some cases later than the feldspar, 

 is mostly of about the same date as the feldspar, and the rock is a gabbro. 



One section. 



Age. Cabotian. 



Remark*. This rock is from one of the Logan sills of the Animikie (see A. C. 

 Lawson, Bulletin viii). In general these sills are diabases, but the section of this 

 rock can properly be called a gabbro. The evidence, however, is not conclusive that 

 it is an apophysis from the gabbro of the great gabbro mass which lies just to the 

 south of this area of Animikie. u. s. G. 



No. 309. SLATE. 



North side of Gunflint lake, about half way from the eastern extremity. 

 Ref. Annual Beport, ix, pages 81, 82; Annual Report, xvi, page 68. 



Meg. An aphanitic, dark, greenish-gray slate or schist. 



Mic. Under a low power the rock shows a few grains and crystals of magiictiti' 

 and pyrite, small specks of an opaque substance which is black in transmitted light 

 and gray in reflected light, and numerous black dust-like particles, all imbedded in a 

 minutely crystalline groundmass. Under a high power, this groundmass appears to 

 be made of minute grains of quartz, scales of chlorite and a few minute Muscovite 

 scales. 



One section. 



Age. Archean (Keewatin). u. s. G. 



No. 310. GRAYWACKE.. 



North side of Gunflint lake, halfway from the eastern extremity; associated with vertical slates into which 

 it graduates. 



Ref. Annual Report, ix, pages 82, 85; Annual Report, x, page 17; Annual Report, xvi, page 68. 



Mac. Firm, harsh, siliceous rock, with grains of free quartz, of a light green 

 color; evidently also contains much feldspathic material. 



Mic. A fragmental, slightly schistose rock, the grains all much altered. 

 Quartz is the most conspicuous element, in sub-rounded grains, some of them 

 being composed of several individual grains, as if from a sandstone or a conglomer- 

 ate originally. Feldspar, clouded and semi-opaque between crossed nicols, but 

 occasionally showing albite twinning, is very abundant, in sub-angular fragmental 

 grains. The matrix for these two consists of finer grains of the same and a little 

 coloring matter such as hematite, biotite and chlorite. The fibrous and laminated 

 minerals are mainly arranged in one direction. 



One section. 



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



