PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 553 



Gneiss. Granite.] 



A partial chemical analysis of this rock gave the following results: 



SiO 2 74.43 



A1 2 O 3 12.68 



Fe 2 3 and FeO 3 . 82 



CaO i.28 



MgO - 0.25 



Na a O 1. 91 



K 2 - 2.25 



Total 9Q.C2 



One section. 



Age. Archean (igneous). u. s. G. 



No. 804. GNEISS (with biotite). 



Granite City, Morrison county. 

 Ref. Annual Report, x, page 107. 



Meg. A medium to coarse-grained granitic rock, composed of white feldspar, 

 quartz and much biotite, the plates of the last mineral giving a gneissic structure to 

 the rock. 



Mic. The section is composed largely of feldspar. This is mainly microcline and 

 orthoclase. All the feldspar is more or less clouded, and there are some small rounded 

 grains of this mineral embraced in larger grains of the same. These smaller grains 

 are so much altered as to be almost opaque. Quartz and biotite are also present. 

 There is a little chlorite, apparently an alteration product from biotite, and in one 

 small area are many muscovite scales. 

 One poor section. 



Age. Archean. u. s. G. 



No. 805. GRANITE (with biotite and hornblende ). 



Watab, Benton county. 



Ref. Annual Report, x, page 107. 



A coarse-grained, pinkish granite composed of pinkish feldspar, quartz, 

 biotite and hornblende. 



.]//>. The sections show much feldspar apparently orthoclase, microperthite 

 abundantly twinned plagioclase near oliyodase, and mi (><></ hie. There are also some 

 quartz, biotite, hornblende and a few apatite needles. The biotite is sometimes in 

 small, irregular areas and shreds, surrounded by areas of minute flakes of the same 

 mineral. Such areas are regarded as secondary. There is a little micropegmatyte 

 of quartz and feldspar. 



Two sections. 



Age. Archean. u. s. G. 



No. 806. GRANITE ( with hornblende and biotite). 



Watab, Benton county. 



Ref. Annual Report, x, page 107; Final Report, vol. i, pages 144, 145, 196-199 (No. 10). 



