618 THE GEOLOGY OF MINNESOTA. 



[Magnetite, olivine, quartz. 

 Gabbro. Dioryte. 



Mic. The section is similar to those of Nos. 973 and 974, but in this rock (No. 

 975) the feldspar is more abundant than in the other two. 

 One section. 

 Age, Archean. u. s. G. 



No. 976. MAGNETITE, OLIVINE, QUARTZ. (Ferruginous muscovadyte. ) 



From a boulder a quarter of a mile south of the second trail-crossing of Dunka river; S. W. % sec - 10> T. 

 62-12. (Compare Nos. 960 and 1138.) 



Ref. Annual Report, xv, pages 341, 392; Annual Report, xix, pages 121, 127. 



Meg. Black, magnetic, with quartz and olivine apparent. 



Mic. Quartz, olivine, magnetite and griinerite (?) make up this rock, magnetite 

 being most abundant and olivine next, all being of secondary generation from the 

 jaspilyte lodes of the Keewatin when impure with intermingling of basic sedimentary 

 material, the recrystallization having resulted from the action of the gabbro. In the 

 olivine of this section is a good illustration of a fact which is not uncommon in the 

 generation of secondary minerals, viz.: The larger olivines often embrace a number 

 of fine globular grains of the same mineral, as illustrated by figure 6, plate II. 



Age. In the annual report this ore was placed as the bottom of the Animikie, 

 but is now believed to be a modified condition of the Keewatin ores, always accom- 

 panied by muscovadyte. 



Remark. Although the specimen was obtained from a boulder, the rock is in 

 situ in the immediate vicinity. 



According to Hintze the griinerite, determined by Lane and Sharpless, should 

 have been called cummingtonite (Handbuch der Mineralogie, Bd. II, page 1230). 

 Examined by Prof. Dodge this ore gave no titanium. The structure of this rock is 

 illustrated by figure 5, plate II. N. H. w. 



No. 977. GABBRO. (Olivinitic. ) 



S. W. J^ sec. 10, T. 62-12 W.; at the second crossing of the river by the trail which runs from Birch lake 

 southward along Dunka river. 



Ref. Annual Report, xv, pages 341, 392. 



Meg. A fine-grained, yellowish-gray, granular rock. 



Mic. The section shows a granitoid rock consisting chiefly of plagioclase, 

 pyroxene and olivine. At least some of the pyroxene is orthorhombic, apparently 

 hypersthene. Magnetite and biotite are common. 



One section. 



Age. Cabotian. u_ S- G 



No. 978. DIORYTE. 



S. E. % sec. 28, T. 61-12 W.; Birch lake. 

 Ref. Annual Report, xv, pages 340, 392. 



Meg. A medium-grained, dark-gray rock. 



