372 THE GEOLOGY OP MINNESOTA. 



[Quartz. Argillyte. Graywacke. 



Meg. A dark-gray, almost black, rock, having a rough cleavage. It is con- 

 siderably coarser grained than most slates, but the constituent minerals are not to 

 be determined macroscopically. It has a much rougher texture than the usual black 

 slates of the region. 



Mic. The section is composed essentially of dolomite and a black finely dissem- 

 inated material which may be carbonaceous. There are also a few minute grains of 

 quartz, and possibly some feldspar, and some flakes of chlorite. The dolomite is the 

 main constituent of the rock. It appears like calcite, but is called dolomite, because 

 the rock effervesces very slowly in cold acid, but rapidly when warm. In seams in 

 the hand specimen the same mineral is developed. 



One section. 



Age. Animikie(?) u. s. G. 



No. 454. QUARTZ. (Vein.) 



Same locality as No. 453. 



Ref. Annual Report, x, pages 12, 21. 



Meg. A mass of coarsely crystallized milk-white quartz. There are also pieces 

 of what might be the wall rock of the vein. These are quite soft, and green or 

 greenish gray in color. A little dolomite is also scattered through the specimen. 



No section. 



Age. Vein in Animikie(?) rocks. u. s. G. 



No. 455. ARGILLYTE. 



Lower falls of the St. Louis river. Embraced between and contiguous to white quartz veins and laminae. 

 Ref. Annual Report, x, page 12. 



Meg. This is a fine, nearly black clay state, with numerous joints and plications 

 formed by pressure and slight shearing, apparently sparsely garnetiferous. 

 No section. 

 Age. Animikie(?) N. H. w. 



No. 456. GRAYWACKE. 



Embraced in the slate at the lower falls of the St. Louis river. 

 Ref. Annual Report, x, page 12. 



Meg. This is a typical graywacke. The grains of quartz are abundant, their 

 larger diameters sometimes reaching one half a millimeter. The feldspars are 

 somewhat kaolinized and rarely of a flesh-red color, and would average perhaps a 

 little larger than the quartzes. These are embraced in a fine matrix of greenish, 

 perhaps chloritic and siliceous matter, which gives color to the rock, which is dense, 

 fresh and firm. It has a coarse, oblique structure, hardly susceptible of being called 

 schistosity, which is coincident with the slatiness of the contiguous argillyte and 



