554 THE GEOLOGY OP MINNESOTA. 



[Quartzyte. Granite. 



Meg. A coarse-grained, gray granite composed of white feldspar, quartz, biotite 

 and hornblende. 



Mic. Feldspar, quartz, hornblende, biotite, chlorite, as an alteration product of 

 hornblende and biotite, and magnetite. Sphene is common, and there is a little epidote. 

 The feldspar is ortltodase, microperthite and p/agioclase. The last is in great quantity, 

 and is abundantly twinned, in some cases the twinning being exceedingly minute. 

 This plagioclase is probably both oligochtse and anorthoclase. 



A chemical analysis of this rock gave the following results: 



SiOj 78.12 



AljO, 11.14 



Fe 2 O 3 and FeO 2.68 



CaO 0.62 



MgO trace 



Na 2 O 3.33 



K,0 4.48 



H,0 - 0.43 



Total 100.80 



Two sections. 



Age. Archean. u. s. G. 



No. 807. QUARTZYTE. 



Duluth. 



Ref. Annual Report, x, pages 107, 108. Compare rocks Nos. 44, 1966 and 1967. 



Meg. A compact, very fine-grained, dark-gray to black, reddish-weathering, 

 almost flint rock. This rock underlies the diabase No. 43, and in the field appears 

 to be a part of No. 42. It overlies No. 808. 



Mic. The section consists of small, irregular grains of quartz, and a red, cloudy 

 feldspathic substance which acts almost as a cement to the quartz grains. A little 

 chlorite and magnetite are present. 



One section. 



Age. Animikie. 



Remarks. This rock resembles some of the very fine-grained, metamorphosed 

 quartzytes or quartz slates of the Animikie at Pigeon point. u. s. G. 



No. 808. GRANITE. 



Newson's old quarry, Duluth. Underlies No. 807. 

 Ref. Annual Report, x, pages 108, 109. 



Meg. A medium-grained, reddish, granitic rock, composed mainly of reddish 

 feldspar and quartz. 



Mic. The section shows one of the usual "red rocks." There is much quartz 

 and clouded and reddened feldspar, the red color being due to the presence of much 

 finely divided hematite. Chlorite is present, also a little epidote. One section. 



Age. Cabotian. u s G 



