508 THE GEOLOGY OF MINNESOTA. 



[Gabbro. 



The lighter areas in the section are of irregular and not sharply defined outlines, 

 and each area is not always of one grain. The mineral is somewhat cloudy, and not 

 completely transparent; this appearance may be due entirely to the numbers of 

 inclusions it contains. It has approximately the index of refraction and the double 

 refraction of quartz. No cleavage is to be seen. The interference figures obtained 

 were very indistinct. A few were seen which are of a uniaxial, or closely uniaxial 

 mineral, and the character of the double refraction is negative. This mineral may be 

 cordierite, 



One section examined. 



Aye, Animikie. r - s - (: - 



No. 713. GABBRO (wit It 



Nort.li shore of Tucker lake. This lake lies in sees. 2, 3 and J, T. (U 3 \V. This embraces also masses of 

 rock like No. Td-J. 



Ref. Annual Report, x, page 84; Bulletin ii, page 71, plate I, figure 2. 



Mi, i. A coarse-grained gray rock, composed of feldspar, angite, magnetite, 



biotite, hornblende and oliviue. 



Mii: The section shows a considerably altered oliviue gabbro. The original 

 minerals are plagioclase, augite, olivine and nuit/ndHc. Contrary to the usual order 

 of alteration, this section shows the plagioclase and the augite frequently very 

 highly altered while the oliviue is usually comparatively fresh. The plagioclase has 

 changed largely to " saussurite " which in this case consists of cliloritc, zoisite and 

 colorless micaceous minerals. The zoisite commonly shows the blue interference 

 color which is so frequently given by this mineral. The augite has in many places 

 changed to a mass of green fibrous lionili/nuli', <-hl<>ritt\ magnetite, biotite and a little 

 brown liornlileinlc. 



One section examined. 



Aye. Cabotian. r. s. G. 



No. 714. GABBRO (n-it/i ln-inc). 



From the hill range south of Tucker lake, in the north part of sec. 10, T. 64-3 W. 

 Ref. Annual Report, x, page 84; Bulletin ii, page 03. 



Mnj. A medium-grained, granular, dark-gray rock, composed almost entirely 

 of plagioclase and augite. 



Mic. The section shows a gabbro of finer grain than the main mass of the 

 gabbro of northeastern Minnesota. The minerals are />//n/ii>c/(i^', <ui</it< j , magnetite, a 

 little olivine, a few small flakes of f>iofif<>, and alteration products. 



One section examined. 



Age. Cabotian. u. s. G. 



