652 THE GEOLOGY OP MINNESOTA. 



[Tuff. Graywacke. 



pyrite and siderite, the latter frequently surrounded by a rustiness due to its 

 oxidation. In one case the pebble contains old feldspars of larger size, recalling the 

 rocks of Kekequabic lake, but as the pebble has no porphyritic hornblendes nor 

 secondary growths, but much scattered chlorite and calcite, it is hardly to be referred 

 to that origin, but may be considered a case parallel with that mentioned under 

 No. 1065. One section. 



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



No. 1071. TUFF(?) 



Northward projecting point south shore of Frog Rock lake; sec. 18, T. G.~i ."i. 

 Ref. Annual Report, xv, pages 372, 396. 



Mey. Porphyritic, apparently, with some pyroxene, but generally a green, 

 amorphous rock. Contains pyrite. 



Mir. Throughout the matrix, which is fine and chloritized and nearly dark 

 between crossed nicols, are disseminated many fragments of uralitized augite and of 

 feldspar. There are but few hornblendes that show secondary growths of aciixoUte. 

 Epidote also is conspicuous. There are areas in the slide that are darker and of 

 different grain from the rest, indicating a pebbly or tuffaceous accumulation. One 

 section. 



Age. Archean (Keewatin). 



Remark. This rock seems to be allied in origin and age ^o the Kekequabic Lake 

 fragmental green schists. N. H. w. 



No. 1072. TUFF. (Basic.) 



East end of Frog Rock lake; sec. 17, T. 65 5. 

 Rcf. Annual Report, xv, page 372. 



Mey. Greenstone-like rock. Contains pyrite. 



Mic. Rounded and sub-angular quartzes and feldspars are disseminated as 

 clastic debris in a green calciferous matrix, in which are also leucoxene a,nd pi/rife. 

 In this slide is seen also a 'pebble of diabase which shows yet its ophitic structure. 

 In this pebble the pyroxenic element is altered entirely to a chloritic green mineral of 

 low double ref rction in which is much fine-grained calcite. One section. 



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



No. 1073. GKAYWACKE. 



Mouth of Ogishkie Muncie creek; sec. 26, T. <r> (i. 



Ref. Annual Report, xv, pages 372, 37C, 306, 307; Annual Report, xvii, pages 107, 205. 



Meg. Coarse jointed, massive. 



Mic. Bather quartzose, fragmental, containing also fragments of >nirn>dine, 

 plaffioclase, zircon, mAv'/r. Two sections. 

 Aye. Archean (Keewatin). 



