484 THE GEOLOGY OF MINNESOTA. 



[Granite. 



color so common in the augites of the associated diabases and gabbros. A consider- 

 able part of the augite has altered and among the alteration products hornblende is 

 common. 



Quartz is common and occurs in well-defined grains and also in the micropeg- 

 matyte. Biotite is rare and is secondary. Apatite occurs in needles of considerable 

 size. Areas of brownish semi-opaque alteration products are common; at times 

 these show marked absorption and possibly are of the nature of loirliiujHc. Magne- 

 tite is abundant. 



One section examined. 



Age. Cabotian. 



Remark. See under No. 675. u. s. G. 



No. 675. GRANITE (with hornblende). 



"In the northern face of Brule mountain, 300 feet below the top, about south from the east end of the 

 westerly lake in Brule river. This reddish granitoid rock forms a perpendicular wall of sixty feet in hight." 

 Probably near the southwest corner of sec. 16, T. 63-1 W. 



Ref. Annual Report, x, pages 75, 14'2; American Association for the Advancement of Science, vol. xxx, 

 page 1G5; Bulletin ii, page 81. 



A pinkish-gray rock of medium grain and granitoid texture, composed of 

 pink feldspar, horblende and quartz. 



Mir. The section very closely resembles that of No. 674, except for two partic- 

 ulars: First, the augite is very largely altered and the hornblende is more abundant; 

 and second, much of the feldspar, especially that of the micropegmatyte and the 

 outer parts of the partly idiomorphic individuals, is reddened in a manner similar 

 to the feldspar of most of the red rocks. Much of the larger feldspar individuals 

 show polysynthetic albite twinning and low extinction angles in sections normal to 

 010, or nearly so. One section cut almost exactly normal to a, gave an extinction 

 angle of 78 and two others cut practically normal to a gave angles of 80 and 81 

 respectively. The feldspar in part is thus clearly oligoclase, and no evidence of a 

 more basic plagioclase was seen. One section examined. 



Aye. Cabotian. 



Remark. This rock and the preceding (No. 674) are interesting from the fact 

 that they represent facies of the red rocks which are coarser grained than usual and 

 which appear to be approaching the gabbros. The latter appearance, however, is 

 less evident on a study of the rock. Augite is present in large amounts; thus these 

 specimens differ from the main mass of the red rocks, which, even in their freshest 

 conditions, show a comparatively small amount of augite. There is no necessity for 

 considering the quartz as largely secondary, and the feldspars are clearly alkaline 

 acid species rather than the more basic lime feldspars of the gabbros. There thus 

 seems to be no good reason for regarding these rocks as altered gabbros. u. s. G. 



