PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 379 



Quartz. Graywacke.] 



No. 471. SLATE. (Black.) 



Near the same place as No. 469. From another quarry. 

 Kef. Annual Report, x, page 17. 



Me<j. Almost exactly the same as No. 470, except that it is not quite so black 

 and cleaves a little better. 

 No section. 



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



No. 472. QUARTZ. ( Vein, ) 



Forty rods above the railroad (St. Paul & Duluth) bridge; S. W. % S. W. % sec. 5, T. 48-16 W. The vein 

 coincides with the slaty cleavage of the enclosing slates, but it is irregular and soon pinches out. 



Ref. Annual Report, x, pages 17,18; Annual Report, xviii, page 47; Final Report, vol. iv, page 9 and plate A. 



Mcy. A mass of coarsely crystallized, milk-white quartz. 



No section. 



Age. Vein in Keewatin(?) rocks. u. s. o. 



No. 473. GRAYWACKE. ( Pyritiferous. ) 



From the ridge in which No. 472 occurs. Some of the pyrite cubes are three-fourths of an inch across, 

 and occasionally an inch. 



Ref. Annual Report, x, page 18. 



Mr;/. A fine-grained, hard, siliceous, grayish rock, containing cubes of pyrite 

 which are usually about a quarter of an inch across. The rock is so fine grained 

 that its constituents cannot be determined niacroscopically, but it appears like a fine 

 quartzyte. The weathered surface is almost white, but on a fracture just below this 

 surface the rock is brownish, evidently due to the decay of some iron-bearing mineral. 



Mic. The section shows qtta,rtz,feldspar, .s/Vr/v'/^ (and also calcite), chlorite, sericite 

 and opaque earthy specks. The quartz and feldspar are in grains of various sizes. 

 In places larger grains of these minerals are embedded in a finer grained groundmass 

 which is composed of all the minerals of the rock. There are, however, gradations 

 in size from these larger grains to the smaller ones of the groundmass. In shape 

 these larger grains are roughly rounded to angular, but even the rounded ones do 

 not have sharp boundaries, but interlock with the surrounding grains. These grains 

 appear to be fragmental, but a careful search failed to show any of them which show 

 distinct outlines which had been added to by secondary growths, although there seems 

 to be no doubt but what this process has occurred widely in this slide. The feldspar 

 is often clouded; no careful determinations were made, but it appears to be orthoclase 

 and oligoclase. The siderite is quite common and is frequently accompanied by a 

 brownish stain. Two small prismatic crystals were seen which show a decided 

 absorption of the ray vibrating parallel to the short axis of the crystal and also 

 parallel extinction. These crystals are probably tourmaline. Two sections. 



Age. Keewatin(?) 



