302 THE GEOLOGY OF MINNESOTA. 



[Granite. 



however, only in one of the sections. The other, and the rock sample from which it 

 was taken, do not show any porphyritic structure. The rock is otherwise coarsely 

 and evenly crystalline, gray and gabbroloid in aspect, and not much decayed. 



Mic. The later augite embraces both olivine and augite ophitically. Magnetite 

 and brown and greenish biotite, sometimes in contiguous masses, each color show- 

 ing the darkened aureoles, accompany the augite, and brown-yellow serpentine 

 occupies fissures in the olivine. It is evident that these aureoles are not imperfectly 

 biotitized remnants of chlorite remaining in the biotite mass, since they are equally 

 distinct in both places. Their cause antedates both minerals. 



The earlier augites seem to have been affected, in some cases, in such a manner 

 by the vicissitudes through which they have passed as to become nearly uniaxial. 

 At least, in one grain, cut perpendicular to n s , the optic angle 2 E is so small that the 

 mineral was at first taken to be some other than a monoclinic pyroxene on account 

 of the black cross presented by the interference figure. After more careful exam- 

 ination, it was noticed that the hyperbolas separate about 5. This is an unusual 

 optical anomaly for augite. 



Magnetite has shapes similar to those of the augite, but is occasionally embraced 

 by the latter in the characteristic ophitic manner. 



Quartz appears as isolated grains of good size, and also as pegmatitic filling in 

 some of the plagioclase. 



Two sections. 



Age. Cabotian. 



lien/ a //,-. The brownish substance above referred to as serpentine, which is 

 sometimes brownish red, may be the same that has been alluded to before as a 

 product of a very ferruginous olivine in process of decay. (M. Hannay, Miner- 

 alogical Magazine, vol. i, page 154, 1877.) The methods of distinguishing this mineral 

 from antigorite, and a summary of all its characters, are given by Lacroix ( Mineral - 

 ogie de France, vol. i, pages 442-445.) See under No. 162. N. H. w. 



No. 292. GRANITE (with biotite). 



" The next rock just west of the canoe portage, on the north shore of the peninsula; forms a similar kind 

 of coast; also is heavily jointed and bedded like No. 291, but it is red with orthoclase. The microscope reveals 

 also hornblende and quartz; occasionally, also, is a grain of milk-white, foliated, soft mineral. This is a granular 

 rock, derived from the fusion and crystallization of the associated sedimentary beds. It weathers and parts as 

 if a conglomerate near the water. This rock continues but a short distance, making one blunt point, when the 

 features and color of No. 291 return again. (See the notes on Nos. 004-613.)" S. E. J N. W. % sec. 26, T. 64-7 E.; 

 north side of Pigeon point. 



Ref. Annual Report, ix, page 70; Bulletin ii, pages 81-83. 



Meg. A medium-grained red granite, composed of red feldspar, quartz and a 

 soft dark green mineral, probably chlorite. There are some drusy cavities on the 

 sides of which are crystals of the minerals of the rock and calcite. 



