586 THE GEOLOGY OF MINNESOTA. 



[Pebbles. Greenstone. Dioryte. 



Age. Stuntz conglomerate (base of Upper Keewatin). 



Remark. In some of the slides the rock looks decidedly like a quartz-porphyry 

 somewhat sheared. In fact the rock really is a "recomposed" quartz-porphyry. In 

 connection with Nos. 874 and 874A, compare the descriptions of Nos. 2010 to 2030. 



u. s. G. 

 No. 874A. PEBBLES (from conglomerate. ). 



Pebbles from No. 874. 



Ref. Annual Report, xv, page 387. 



Meg. The pebbles collected are largely of light-gray quartz-porphyry. There 

 are also some pebbles of different facies of jaspilyte. 



Sections from part of the matrix attached to one of the pebbles are 

 similar to sections of No. 874. Sections from the pebbles show a porphyritic rock, 

 the phenocrysts being of considerably altered feldspar, and the groundmass consisting 

 largely of trachitic feldspars, with apparently some quartz, and flakes of Muscovite. 

 Calcite is quite common. 



Four sections. 



Age. Pebbles in Stuntz conglomerate (base of the Upper Keewatin). u. s. G. 



NO. 874B. "GREENSTONE." 



"Olivinitic greenstone, found in No. 874, near some quartz veins, Stuntz's island, sec. 21, T. 62-15." 

 Ref. Annual Report, xv, pages 310, 387. 



Meg. Specimen not found. 

 No section. 



Age. Keewatin. I T - s. G. 



No. 875. DIORYTE. 



Dike in southern part of sec. 12, T. 62-15 W.; east end of Vermilion lake. 

 Ref. Annual Report, xv, pages 306, 387. Compare Nos. 872 and 873. 



Meg. A medium-grained, dark-gray, granitic rock, composed of hornblende and 

 white to flesh-colored feldspar. One end of the specimen is much finer grained than 

 the main part of the rock, but is apparently of the same composition. There is a 

 pretty sharp contact between the coarser and finer grained parts of the specimen. 



M'n-. The sections were made from the coarse part of the hand sample. The 

 rock is composed essentially of hornblende and feldspar, with a little quartz. The 

 hornblende is brownish to green to almost colorless. The brownish hornblende is 

 more massive than the other, which is fibrous. In many cases a brownish grain or 

 one of the darker green grains has a border of a lighter color, the border being 

 optically continuous with the central part of the grain. The feldspar is considerably 

 altered and clouded and affects polarized light rather weakly. In general it is not 

 twinned, but frequently shows perthitic intergrowths. In a few places it is inter- 



