PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 628 



Gneiss.] 



Mic. The section shows a granitoid rock in which the chief mineral is feldspar; 

 this is considerably kaolinized, and sometimes shows albite twinnings, but much of 

 it is too far altered to exhibit these twinnings, even if they originally existed. In 

 addition to the feldspar, quartz, chlorite and epidote are seen. 

 One section. 



Age. Archean. u. s. e. 



No. 990. GNEISS. 



N. W. % sec. 27, T. &3-10 W.; shore of the Kawishiwi river north of No. 989. 

 Ref. Annual Report, xv, pages 352, 392; Bulletin vi, pages 68, 421. 



Meg. A hard, fine-grained, creamy-white rock. 



Mic. The section shows a granitoid rock composed essentially of the following 

 minerals: quartz, feldspar, much clouded by kaolinization, and epidote. 

 One section. 

 Age. Archean. u. s. G. 



No. 991. GNEISS. 



N. W. y N. W. M sec. 27, T. 63-10 W.; island in the Kawishiwi river. 



Ref. Annual Report, xv, pages 352, 353, 392; Annual Report, xvii, pages 195, 206; Bulletin vi, pages 68, 421. 



Meg. A gray, red-weathering, fine-grained, lightly feldspathic gneiss. 



Mic. The sections show a granitoid rock in which feldspar is the chief constituent, 

 and quartz is less abundant. The feldspar is usually considerably clouded by kaoli- 

 nization and is plagioclase and apparently orthoclase. Epidote, green hornblende and 

 chlorite are the chief accessory minerals. There is one small crystal, perhaps zircon, 

 surrounded by hornblende. Two sections. 



Age. Archean. u. s. G. 



Remark. The decayed feldspars of Nos. 989, 990 and 991 are in remarkable con- 

 trast with the other minerals. They never interlock, although in contact. They are 

 surrounded, more or less, and sometimes reconstructed, by secondary granular quartz 

 and feldspar. Occasionally the green hornblende also surrounds small grains of 

 decayed feldspar, and otherwise shows its date of origin was later than the feldspars; 

 but other hornblendes were coeval with or earlier than the feldspars. In several 

 respects these rocks resemble the granites of Kekequabic lake. One hornblende 

 (which surrounds a small zircon) is bounded by the prism faces and is twinned. It 

 is nearly a perfect crystal and must date (as augite) from the earliest condition of 

 the rock. A little of the feldspar, some of the hornblende, all of the epidote and all 

 of the quartz are secondary. The earliest condition of these rocks probably embraced 

 feldspar and quartz chiefly, with a little augite and zircon, and perhaps horn- 

 blende, constituting a graywacke. The fine, fragmental, weathered surface of this 

 rock (No. 991) is shown by photograph in plate II, figure 7. N. H. w. 



