1002 THE GEOLOGY OP MINNESOTA. 



[Muscovadyte. Greenwacke. Granite. 



No. 847G. MUSCOVADYTE. (Fragment in gabbro. ) 



S. W. )4 N. W. % sec. 6, T. 64-5. Island in Gabernichigama lake. 



Ref. Annual Report, xxi, page 65 ; Annual Report, xv, pages 171, 172 ; Annual Report, xxiv, page 127 ; 

 Final Report, v, page 905. 



Meg. Gray, granular, fine grained. 



Mic. Owing to the extreme importance of the characters of this rock in their 

 bearing on the question of the origination of the gabbro, two additional thin sections 

 were made. The rock is distinctly a typical granulitic gabbro, but some of the large 

 diallage masses, having a common orientation throughout, surround granular feld- 

 spars; yet, usually, the diallages are in isolated, small, roundish grains lying between 

 the feldspars, or breaking the margins of two contiguous feldspars. Earely a small 

 diallage is embraced within a feldspar. Two sections. 



Age. Cabotian (changed Keewatin). N. H. w. 



No. 837G. GREENWACKE. ( Tuffaceous. ) 



S. W. J^ N. W. J^ sec. 12, T. 65-5. Shore of Cucumber island in Sea Gull lake. 



Ref. Annual Report, xxi, page 64 ; Annual Report, xxiv, page 124. Compare rock No. 597W, Annual 

 Report, xvi, pages 297-299. 



Meg. A light-greenish, much decayed rock, with numerous subangular included 

 rock masses, which are of lighter or of darker color. The weathered surface carries 

 a rusty film, which is in some cases a sixteenth of an inch in thickness, evidently 

 produced by the oxidation of the rock itself. Apparently tuffaceous. 



Mic. The slide is largely isotropic. The rather radial arrangement of some of 

 the much-altered older crystals suggests an originally igneous massive rock, at least 

 for portions of the rock. These older crystals appear to have been mainly feldspars, 

 and they are now replaced principally by calcite, chlorite, and rarely in part by biotite, 

 but they are occasionally partly preserved. The biotite is interleaved with chlorite. 

 The general isotropic aspect is due to the dissemination of chlorite rather than to 

 the present existence of tuffaceous glass. The rock may have been a microlitic 

 zirkelyte, passing locally into a fragmental tuff. Two sections. 



Age. Probably Lower Keewatin. 



Remark. It is in the midst of such rock as this that occurs the segregated 

 marble of Ogishke Muncie lake. The fresh red mineral mentioned by Dr. Grant 

 (Twenty-fourth Annual Report, page 124) is calcite, effervescing rapidly in cold 

 HC1. N. H. w. 



No. 1030G. GRANITE (with Jwrnblende and biotite ). 



Forms the barrier at the falls of Rainy river at Koochiching. 



Ref. Annual Report, xxiii, pages 55, 221 ; American Geologist, vol. xx, pages 293-299, November, 1897. 



Meg. Medium grained, with much biotite and hornblende, causing a dark 

 speckled appearance, sometimes rather porphyritic. "It contains many dark masses, 



