294 THE GEOLOGY OF MINNESOTA. 



[Diabase. Quartzyte. 



No. 269. DIABASE. 



" From Island No. 2, being the easterly of the first two islands near the coast; a porphyritic doleryte, the 

 larger crystals being of a trielinic feldspar. The whole rock is gray, and has small grains of pyrite, The whole 

 island is formed by a dike of No. 269, flanked by a little quartzyte and slate near the water. The dike is about 

 Hfty feet wide, and the island is not much more." This island is at the S. E. corner of sec. 31, T. 64-7 E. 



Ref, Annual Report, ix, pages 64, 65, 60; Bulletin ii, pages 47 (as No. 169), 118. 



Meg. A gray diabase of medium grain. A few porphyritic plagioclases are 

 present. One of these is three-fourths of an inch across, but the rest are much 

 smaller. A little pyrite is present. The 'feldspathic part of the rock is gray and the 

 rest of the rock is darker and apparently decayed. 



Mic. A much-altered diabase. The original minerals are plagiodase, augite, 

 magnetite and apatite. The first two are much changed. The secondary minerals are 

 hornblende, chlorite, hiotlte, magnetite, pi/rite, (/uartz and a brown almost opaque 

 material. The feldspars contain flakes of a micaceous (kaolin?) mineral. 



Two sections. 



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



No. 270. QUARTZYTE. ( Graphitic. ) 



Pigeon point, S. W. J^, sec. 32, T. 64-7, nearly on the axis of the peninsula. (Compare No. 552.) This 

 rock, at large, is charged with graphite. Some pieces twelve inches and more in diameter have been extracted 

 in the shallow working which has been accomplished. The rock also contains a little native copper and pyrite. 

 It embraces also irregularly angular patches of quartzyte. The graphite occurs most plentifully in the quartzyte, 

 over a belt twenty to thirty feet wide. 



Ref. Annual Report, ix, pages 62, 65; Annual Report, x, page 48; Bulletin vi, pages 123, 420. 



Meg. A gray, medium-grained, irregular rock, rusty, yet sparsely specked with 

 pyrite, and giving the dark metallic lustre of graphite, which seems to be dissemi- 

 nated throughout it. It is evidently the product of the mutual reaction of the 

 sedimentaries on the basic eruptives of the region, and it is, in many cases, hence, 

 not possible to state the greater alliance of a hand specimen, whether with the 

 sedimentaries or the eruptives. Yet, this has uniformly been considered as a modified 

 portion of the quartzytes and slates of the Animikie of the region. 



Quartz is abundant, probably the most abundant element of the rock, though it 

 hardly constitutes one-half. It is all in secondary form, usually in angular grains, 

 not generally pegmatitic, but micro-granulitic. 



Biotite is common, some of the larger plates being brown and giving a uniaxial 

 interference figure. When cut perpendicular to the cleavages, or at least when not 

 parallel to them, this mineral is alternately brown and light green, or greenish brown 

 when, with one nicol in use, it is rotated on the stage. The brown color appears 

 when the direction of the edges of the cut plates agrees with the principal section of 

 the nicol in use. When it is not brown and is cut oblique to the cleavage, it gives 

 colored polarization, approaching that of muscovite. 



