786 THE -GEOLOGY OP MINNESOTA. 



[Gneiss. Quartzyte. 



No. 1518. GNEISS. 



Morton quarries, Morton, Minnesota valley. 

 Ref. Annual Report, xviii, page 60. 



Meg. Gneissic, with much red orthoclase. 



Mic. Orthoclase, oligodase, microdine, quartz, essentially compose this rock. 

 The feldspar grains are all affected uniformly by a later process of decay which 

 has given rise within them to scattered scales of calcite and miiscovite. There are 

 areas and grains included within the larger feldspars which show much more decay 

 than the rest, but there is no marked general separation either in position or in 

 stage of alteration between the fresh feldspar and the old. One section. 



Age. Archean. N. H. w. 



No. 1519. HORNBLENDE BIOTITE GNEISS. 



Morton quarries. From a dark, nearly black, inclusion in the body of the rock. 

 Ref. Annual Report, xviii, pages 24, 60. 



Mey. Medium grained, micaceous. 



-1//V. Hornblende and biotite give the dark color to the rock. The rock is very 

 fresh and clean. The feldspars are conspicuously twinned on the albite and peri- 

 cline plans and occasionally zoned, but they contain frequently central cores so 

 much altered that the twinning bands are destroyed. The twinning striations do not 

 continue through the new zonal growths. In such cases it is possible that the fresh 

 portion is a secondary growth about an old decayed crystal. But very little quartz is 

 seen. One section. 



Age. Archean. N. H. w. 



No. 1521. QUARTZYTE. 



Pokegama falls, Mississippi river, Itasca county. 

 Ref. Annual Report, xviii, page 60. 



Meg. Gray, granular. (Compare Nos. 1525A, 1526 and 257H-259H.) 

 . Mic. The grains have been enlarged by interstitial growths, forming a solid 

 quartzyte. The grains almost without exception are loosely charged with minute 

 acicular trichiths which are only visible when the quartz is extinct and in high 

 power; they also extinguish parallel with the threads. If these be not due to an 

 accidental impurity in the balsam, they are a feature of quartz not before noticed 

 in the state. One section. 



Age. Pokegama (bottom of the Animikie). N. H. w. 



No. 1525(a). QUARTZYTE. 



Pokegama falls. 



Kef. Annual Report, xviii, page GO. 



Meg. Spottedness shown in the quartzyte. One of the spots was in the centre 

 of the chip from which the section was made. 



Mic. There is nothing in the appearance of the slide which will explain the 

 spottedness unless it be a greater amount of dirty impurities which seems to prevail 



