330 THE GEOLOGY OP MINNESOTA. 



[Sericite schist. Quartz. 



No. 351. SERICITE SCHIST. 



"About three-fourths of a mile above the rapids the slates dip northwest." Newton lake; probably in N. 

 E. 1^ sec. 27, T. 64-11 W. 



Ref. Annual Report, ix, page 91. Same as No. 257 (W.), Annual Report, xv, page 104. 



Mc(j. A very fine-grained, light greenish gray, rather fissile schist. Occasionally 

 a small grain of feldspar can be seen, otherwise the rock is almost aphanitic. 



Mic. The schistose structure is finely shown in the section ; there are irreg- 

 ularly parallel streams of fine gray opaque material, and the scales of wr/cite and 

 chlorite are elongated in a common direction. The rock consists of sericite, chlorite, 

 quartz, feldspar, the gray opaque substance, calcite and a brownish stain. The sericite 

 (or what is so regarded) and the chlorite are in minute flakes or scales; the former 

 polarizes brightly. With these scales is a very fine-grained aggregate of (jnnrtz and 

 apparently feldspar also. The section shows also larger grains of feldspar, which are 

 broken and show undulatory extinction. Usually several grains of almost parallel 

 orientation, and separated by growths of sericite, are close together, suggesting 

 that the several grains are from one broken crystal. In this respect they are 

 closely similar to the " stretched " grains of feldspar figured by Gr. H. Williams in 

 Bulletin Ixii, U. S. Geol. Survey (figure 2, plate IX; figure 2, plate XIV). These 

 feldspars very rarely show twinning; they are somewhat altered, sericite being 

 developed in them, and do not as a rule show cleavage, so their species was not 

 determined. They are, however, probably orthoclase. The opaque gray material, 

 spoken of above, under a high power is seen to be made of minute highly refractive 

 grains and crystals which seem in part to be epidote. One section. 



Age. Archean (Keewatin). 



Remark. What this rock was originally cannot be determined from the single 

 specimen and section at hand. It may not be amiss, however, to suggest that it 

 represents an advanced stage of shearing and stretching in a quartz-porphyry or 

 similar rock, the broken and almost obliterated feldspars being the only original 

 grains left in the rock. It may also be a sheared debris derived largely from quartz- 

 porphyry, u. s. G. 



No. 352. QUARTZ. (Vein.} 



"At the upper end of the second rapids, or a little distance above, near the portage landing, is a large 

 white quartz vein in the chloritic rock that makes the rapids. This runs S. 30 W., and coincides with the slate 

 in dip, which is toward the northwest. This quartz embraces rusted pyrite, and has an auriferous aspect." 

 Near the S. E. y sec. 22, T. 64-11 W. 



Ref. Annual Report, ix, page 91. This vein is illustrated by figure 1, plate AA, vol. iv. 



Meg. Milk-white quartz enclosed in a green, fine-grained chloritic schist. The 

 schist contains rusty spots probably due to the decay of a ferruginous mineral pyrife. 

 No section. 

 Age. Vein in Archean (Lower Keewatin) rocks, u. s. o. 



