364 THE GEOLOGY OF MINNESOTA. 



[Magnetite. 



Mic. The slide shows microcline, a cloudy feldspar which shows neither cleavage 

 nor twinning and is presumed to be orthoclase, quartz, hornblende, magnetite and a 

 single compound grain, not twinned, of a biaxial, light yellow, rather highly polar- 

 izing and refractive mineral which has the characters of augite, except that the light 

 yellow color is more like that of epidote. Its extinction is at a position which 

 bisects the angle between the principal cleavages, and when the cleavages are vertical 

 to each other its extinction is parallel to the cleavages. This occurs when the 

 section is cut perpendicular to the plane of symmetry (compare No. 133). This 

 mineral is always in association with some of the hornblendes, and is sometimes 

 seen in remnants in the centres of the larger hornblendes, indicating that it has been 

 altered largely to hornblende. In some instances there is a light-colored periphery 

 separating it from the hornblende which extinguishes with the hornblende, but has 

 a low bi- refraction ; indeed, as low as the white of the first order. The hornblende is 

 light green and distinctly pleochroic; it often embraces sphene, which is also quite 

 common elsewhere in the form of small isolated sub-angular yellowish grains. 



Two sections. 



Age. Archean. 



Remark. This granite resembles the granite of Kekequabic lake in several 

 respects. The clouded feldspars above mentioned are the central areas of feldspars 

 which, about their borders and often running all through them, have fresh growths. 

 This might be called a zoning, when casually examined, but it is not due to zonal 

 accretions about a feldspar floating in a magma. It is due rather to the regeneration 

 which has penetrated a granitic debris. This rock, therefore, is to be considered a 

 recomposed and recrystallized granite. * N. H. w. 



No. 436. MAGNETITE. 



Near east line of sec. 15, T. 59-14 W. 

 Kef. Annual Report, ix, page 108. 



Meg. Fine-grained,, glistening magnetite, probably containing some quartz or 

 other siliceous mineral. No section. 



Age. Animikie. u. s. G. 



No. 437. GRUNERITE-MAGNETITE ROCK. 



Same locality as No. 436. The rock associated with the ore. 



Ref. Annual Report, ix, page 108; Annual Report, xvi, page 72; Bulletin vi, pages 119, 123, 129, 420. 



A fine-grained brownish-gray rock which becomes lighter or almost white 

 on weathering. It is made up of a mass of small, short, rather fibrous grains and 

 magnetite and effervesces with hydrochloric acid. 



Mic. The section is composed of a light yellowish mineral, magnetite and calcite. 

 The light yellowish mineral is a monolinic amphibole. It is light yellowish to almost 



