538 THE GEOLOGY OP MINNESOTA. 



[Granite. Gabbro. 



No. 771. GRANITE (u-ith biotite). 



Near the west end of East-and-West lake, on the south shore; probably in S. E. J^ S. W. J4 sec. 15, T. 

 04- 5 W. From a vein twenty inches wide and dipping 75 towards the east, cutting the gabbro. 

 Ref. Annual Report, x, pages 98, 99. 



Met/. A medium-grained reddish granite, composed of colorless to red feldspar, 

 quartz, a little biotite and also apparently a little hornblende. 



Mic. The section shows feldspar, quartz, biotite and a little chlorite and >nu</- 

 nt'lHc. The feldspar is in general some clouded and in places is much reddened. It 

 seems to be largely orthodase, but there are some grains which show very minute 

 twinning lamellae, which are usually visible only near the point of extinction, 

 probably anorthodase. 



One section examined. 



Aye. Cabotian. u. s. G. 



No. 772. GABBKO (icitk Iwrnblcnde ami biotite). 



Lake in Frog Rock river. This lake is probably the one at the northwest corner of sec. '2G, T. 0-1 ."> W. 

 From a vein one foot wide. 



Ref. Annual Report, x, page 99; Bulletin ii, pages 75, 70. 



Meg. A coarse-grained, granitoid rock, composed of feldspar, hornblende, biotite, 

 and magnetite. 



Mic. M. E. Wadsworth's description of this section is as follows:* 



"The section shows a partially altered plagioclase, biotite, quartz, ciriili/i- and 

 magnetite. Of these the quartz, biotite and viridite are secondary or alteration 

 products. Another section shows some diallage, with green and brown hornblende 

 and biotite as associated alteration products of the diallage. The section contains 

 secondary quartz and titanite, and much of the rock is a typical quartz-dioryte formed 

 by the alteration of a gabbro." 



Two sections examined. 



Age. Cabotiau. u. s. G. 



No. 773. GABBKO. 



Last lake in Frog Rock river; probably in S. E. y sec. 27, T. 64-5 W. 



Ref. Annual Report, x, pages 99-101, 103; Bulletin ii, page 71, plate IV, figure 2. 



Mcy. A coarse-grained, granitoid rock, composed largely of feldspar, with some 



augite and magnetite. 



Mic. M. E. Wadsworth's description of this section is as follows :f 



" Its pyroxene constituent is seen under the microscope to be altered to viridite 



and a greenish and brownish biotite. The plagioclase is somewhat changed and 



contains secondary orthodase, quartz, mica scales, etc. 



^Bulletin ii, pp. 75, 76. 

 ^Bulletin ii, p. 71. 



