314 THE GEOLOGY OF MINNESOTA. 



[Granite. 



Meg. This is similar to the rock No. 305, but has generally less amphibole and 

 more quartz. 



Mic. The thick section is composed of quartz with shadowy extinctions, a 

 clouded /'r/V.s^w that cannot be determined, chlorite, epidote and a little hornblende. 



One section. 



Age. Archean (igneous). N. H. w. 



No. 316. GRANITE (withliornblende). 



Saganaga lake, N. E. % sec. 4, T. 66-4 W. A few rods below the rapids. 

 Ref. Annual Report, ix, page 84; Annual Report, x, page 106. 



Meg. A rock in general similar to the last, but having large quartzes. 



Mic. Consists of much quartz, nticrodine, <>rthoclase( ?) and ol'xjochise. The last 

 is indicated by the fine, uniform striations, and the constant parallel, or nearly parallel, 

 extinction. At the same time the striations are not fine enough to be charac- 

 teristic of anorthoclase. The section also shows e/>idofe, chlorite and sphene, the 

 chlorite no doubt a result of decay of the hornblende common in the rock. 



A feldspar grain cut nearly perpendicular to n f has extinction at 75, which 

 agrees tolerably well for oligoclase, according to the table of Fouque.* In general 

 the feldspar is much kaolinized. The quartz grains, which appear larger in the hand 

 sample, are seen under the microscope to consist of many parts, the result of a crush- 

 ing to which they have been subjected. A shadowy extinction passes over them, 

 and many fragments sometimes have nearly the same point of extinction, but so 

 removed that a slight rotation in the same direction brings them successively into 

 darkness. Two sections. 



Age. Archean (igneous). N. H. w. 



No. 317. GRANITE (ivith /tornh/<>,Hle). 



Small island in Saganaga lake, sec. 5, T. 66-4 W. 

 Ref. Annual Report, ix, page 84. 



Meg. A medium-grained, pinkish granite, composed of approximately equal 

 amounts of pink feldspar, quartz and hornblende. There are two small hand samples, 

 one of which has a rude banding due to excess of hornblende in rough layers. 



Mic. A granite composed of feldspar, quartz, green hornblende and some chlorite. 

 The feldspar has a tendency to an idiomorphic form, is much decayed, but sometimes 

 shows traces of fine twinning striae and of zonal structure. When the twinning 

 striae (albite) are present the extinction on either side of the twinning line is low, 

 indicating most probably anorthoclase or oligoclase. There is also a little magnetite 

 and cpidote in the section. One section. 



Age. Archean. 



* Bulletin cle la Societi i/c Miin'ruln/jii' il</ Fmna; vol. xvii, p. 428 (1894). 



