892 THE GEOLOGY OF MINNESOTA. 



[Gneiss. 



not. Alteration to sericite is quite common, and in some cracks and areas of 

 considerable size in the groundmass sericite has been developed in large amount. 

 Secondary calcite is also present. No distinction between the flesh-colored and the 

 blood-red feldspars can be made in ordinary section ,- -they both appear colorless; 

 and as yet no oriented sections of the two kinds have been studied; they appear very 

 distinct in the hand specimen. Scattered through the rock are colorless to greenish 

 secondary needles of chlorite or hornblende. A few areas of green hornblende, more 

 or less altered, also occur, but it is not possible to say whether this hornblende was 

 the form of the original ferro-magnesian constituent of the rock. Sphene and apatite 

 are present in small amount. Large quartz grains are not present in the sections 

 examined, although macroscopic grains are often seen in the hand specimens collected. 

 One section. 



Age. Archean. u. s. G. 



No. 407G. GNEISS. (Biotit-ic.) 



S. E. % N. W. 14 sec. 28, T. 63-10 W. South of the Kawishiwi river. 

 Ref. Annual Report, xx, pages 49, 99. 



Meg. Medium grained, gray, showing biotite. 



Mic. This is like a regenerated graywacke. It contains many old, centrally- 

 clouded feldspars, most of which have not been much extended, or not at all, beyond 

 their original limits, by new growths, but some of which have been thus enlarged, 

 and they have probably all been more or less reformed by the metamorphosing 

 action which the rock has suffered. This metamorphosing action has wholly 

 reformed the quartz, some small granules of which are embraced within the 

 regenerated feldspars, but the most of which forms the interlocking plexus in which 

 the feldspars lie. Besides biotite, the rock contains epidote, broirn horti/>letnle( ? ) and 

 chlorite, these forming part of the groundmass; also muscovite (generally within 

 the feldspars) and a little apatite and sphene. One section. 



Age. Archean (modified Keewatin). 



Remark. Neither by its mineral composition nor by its internal structure can 

 this rock be distinguished from many granites, nor from the modified graywackes, 

 such as Nos. 3S5G and 386G. N. H. w. 



No. 408G. GNEISS. 



Same place as No. 107G. 



Ref. Annual Report, xx, pages 49, 98. 



Meg. Gray, medium-grained, typical gneiss. 



Mic. The same barred structure of some of the large feldspars mentioned 

 in the description of No. 385G, is conspicuous in this rock, and the bars some- 

 times have two directions, in the same crystal, crossing each other at an 



