502 THE GEOLOGY OP MINNESOTA. 



[Cumberlandytr. 



hornblende. It is less abundant than the pyroxene. It is likely to be flecked by 

 decay, the colored polarization indicating the formation of mica scales, amongst 

 which zoisite is occasionally found. 



The p;/ru.cene is usually in the form of tlinflai/i 1 . It is not separated usually 

 from the magnetite by any rim of hornblende, but it has an abrupt and clear contact 

 with it. Yet in one place in the section such a rim separates the diallage from the 

 magnetite. The umi/i/rtifi.', judging from the lack of leucoxene, is but little 

 titaniferous. 



That which is here called boirlimjitc is the same that Wadsworth (Bulletin iii, 

 Minnesota Geological Survey) stated is " a substance of unknown character." It was 

 named by Hanuay (Mineralogical Magazine, i, 154, 1877), and has been further eluci- 

 dated by Lacroix in his last work (Mineralogie de France, vol. i, page 442, 1895). It 

 is always a product of decay of olivine rich in iron. In the section examined there 

 are left traces of the olivine in the rims surrounding the feldspars embraced in the 

 magnetite, which are sometimes so intimately associated with this mineral that the 

 transition from olivine to it is evident. 



In several of the gabbros already described the fact that olivine forms a narrow 

 rim surrounding the magnetite grains has been noted (see No. 512). Its partial 

 replacement, in such position, by brown hornblende is also common. The general 

 transition from olivine to bowlingite is the cause of the bowlingite rims about the 

 feldspars in this rock, this transition being simply one of natural decay of the olivine. 



In general, bowlingite takes a wide range in color. According to Lacroix it is 

 dark brown, or reddish brown, with little or no visible cleavage or micro-structure, 

 or it is lighter colored (yet still brown), but laminated as if by cleavage. Other 

 forms are yellowish and greenish, passing even to nearly translucent, though yet 

 greenish, either laminated (similar to pennine) or finely fibrous. It is frequently 

 pleochroic, but not in all cases. Its double refraction is strong, giving colors in red 

 and yellow even in sections less than .03 millimeter in thickness. 



In the section examined this rim is light-yellow in natural light, rarely greenish, 

 becoming translucent as it is replaced by unchanged olivine. 



In one section (that examined by Wadsworth) is much olirine. This is some- 

 times in the form of large grains and sometimes as rims separating the magnetite 

 from the feldspar, giving place occasionally to the rim of bowlingite. 



Besides the bowlingite rim, Prof. Lacroix has kindly called attention to the 

 characters of the brown rims, which are frequently taken for biotite. They have a 

 uniform extinction embracing sometimes the whole border, or a part of a border of 

 a single grain or even the borders of contiguous grains. They do not remain dark 

 on rotation, as biotite would if lying flat in that manner, and parallel to the cleavage. 



