504 THE GEOLOGY OF MINNESOTA. 



[Granite. 



which may be the remains of original crystals, are in short, irregular, ragged crystals 

 and grains considerably altered and belonging to cit*ttifif<>. The feldspar is in part 

 pl<i(jioclae and in part orthoclase, while ntayiti'titf, in part secondary, and secondary 

 /licit if c and quartz are quite common. The rock is metamorphic, but in all probability 

 a metamorphosed eruptive instead of a sedimentary one." 



The texture of the rock resembles that usually seen in granites in the fact that 

 the constituent grains vary considerably in size, there being many minute ones. 

 These are, however, not due to dynamic action after the crystallization of the rock. 

 The associated gabbros almost always have the constituent grains of approximately 

 uniform size throughout a given section. The feldspar is both untwinned and 

 twinned according to the albite law. Extinction angles show that the mineral is 

 in most part near an<Icsinc- and labradorite. The untwinned grains appear to be of 

 the same species as the twinned; an untwinned grain cut almost normal to c. gave 

 an extinction angle of 28, indicating rather basic lalira<loritc. Feldspar more acid 

 than andesine may be present, but it was not determined. The feldspar is in places 

 altering to knolhtite. and chlorite. A careful search failed to reveal any quartz, 

 although it is possible that some of the smaller grains are of this mineral. 



The pyroxene is clearly orthorhombic. The section is about .(U millimeter in 

 thickness, and this mineral is noticeably, although not markedly, pleochroic, a being 

 very pale yellowish, b pale reddish, and c pale greenish. From its pleochroism and 

 its double refraction, which is considerably higher than the feldspar, it may perhaps 

 be referred to hypersthene rather than to enstatite. It alters to a gray, finely fibrous, 

 brightly polarizing aggregate. 



Considerable pyrite is present in the slide. One section examined. 



Age. Cabotian. r. s. G. 



No. 706. GRANITE. (Fhir <jr<iinL ) 



"Through the iron-rock at the location No. 705 run seams, three to six inches in thickness, of pinkish 

 rock consisting of quartz, orthoclase and hornblende, in rather fine grains. These seams and veins are parallel 

 with the basaltic jointage of the rock No. 705. Sometimes a pinkish shade of color runs into the trap, accom- 

 panied by quartz." 



Ref. Annual Report, x, page 83; Bulletin ii, pago TII. 



Mcy. A fine-grained, granitoid, light-gray rock, of a pinkish shade on weathered 

 surfaces. The minerals are quartz, white feldspar and a small amount of black 

 micaceous substance. 



Mic. The section shows a granitoid aggregate of quartz, fe/</xj>(ir and a little 

 chlorite which in places appears to be an alteration product of biotite. The feldspar 

 is much clouded and altered and some of it is nearly opaque. Tt is probably largely 

 iirtlioclase, but some grains which still show polysynthetic albite twinnings appear 

 to be oligoclase. One section examined. 



Age. Cabotian. i . s. G. 



