514 THE GEOLOGY OF MINNESOTA. 



[Granite. 



Remark. These two rocks (Nos. 726 and 727) are parts of the black slate member 

 of the Animikie. Their spottedness is probably due to the metamorphosing action 

 of the great diabase sill above them. u. s. G. 



No. 728. GRANITE (withhornblewlf). 



From the island in Saganaga lake which is nearest the entrance of the international boundary stream. 

 Near the centre of the E. % sec. 4, T. 6G-4 W. 

 Ref. Annual Report, x, page 88. 



Meg. A coarse-grained, light-gray, granitic rock, composed of white to pinkish 

 feldspar, quartz and hornblende. Some of the white feldspar is plagioclase. A little 

 hiotite is present. 



Mir. The section shows the hypidimorphic-granular texture, the plagioclase 

 feldspar and the hornblende having a tendency to assume crystal outlines. The 

 minerals are feldspar and quartz in large amount, hornblende in smaller amount; 

 also small quantities of <-hlori1<\ /'/n'l/of*', nmyiictih', apatite, sphene &ud brownish altera- 

 tion product which is perhaps limonite derived from />//!//<'. 



The feldspar frequently shows considerable alteration to a gray, almost opaque, 

 kaolin-like substance. Frequently the centres of the crystals are more altered than 

 the edges. Some of the feldspar presents the aspect of o>'th(><-l<ise, and some of it is 

 finely twinned according to the albite law. The uniformly low extinction angles in 

 these twinned grains indicate a feldspar near anorthoclase or oli;/or/<n<e. The quart/ is 

 frequently in areas of considerable size, but under crossed nicols these areas break 

 up into several interlocking grains. The hornblende is of the usual green variety, 

 and chlorite and epidote are frequent secondary products from this mineral. The 

 latter mineral (epidote) is also developed in small amount in the feldspar. There is 

 one grain which appears like biotite altering to chlorite. No part of the grain is 

 fresh characteristic biotite, and it is uncertain whether it represents original biotite 

 or an alteration product from hornblende. 



One section examined. 



Aye. Archean, probably of date between the Lower and Upper Keewatin. 



u. s. G. 

 No. 729. GRANITE 



Prom "Caribou narrows" in Saganaga lake, N. E. 14 N. E. % sec. 19, T. (56-4 W. 

 Kef. Annual Report, x, page 88. 



A medium-grained, reddish granite, composed of feldspar, which is gray 

 to reddish (usually the latter), quartz, chlorite and a little epidote. 



Mic. The section shows a granite in general similar to No. 728, but quartz: is 

 not so abundant. The feldspar is rather uniformly clouded and altered, but a consid- 

 erable part of the feldspar is seen to be i>lyim-lxe. The original ferromagnesian 



