894 THE GEOLOGY OP MINNESOTA. 



[Porphyry. Granite. Gneiss. 



No. 490G. PORPHYRY. (Syenitic.) 



West shore of a narrow bay of Snowbank lake, S. E. y S. E. % sec. 27, T. 64-9 W. From a dike cutting 

 diabase. 



Ref. Annual Report, xx, pages 61, 102. 



Meg. This porphyry, has a reddish to purplish aphanitic groundmass, in which 

 are porphyritic crystals of red feldspar and small areas of chlorite. 



Mic. Under the microscope the groundmass is seen to be microgranitic in 

 structure and apparently composed of quartz and feldspar. The feldspar phenocrysts 

 are more or less altered and the majority of them show polysynthetic twinning. 

 Irregular areas of chlorite occur in the groundmass, but nothing is left to show what 

 was the mineral that originally occupied these areas. A few small apatite prisms 

 are present, and scattered through the whole rock are minute green flakes of chlorite. 

 Two sections. 



Age. Archean. u. s. G. 



No. 491G. GRANITE. 



West shore of Snowbank lake, at the line between sees. 26 and 35, T. 64-9 W. 

 Ref. Annual Report, xx, pages 62, 102. 



Meg. It is a granite of medium grain, reddish color and compact texture; the 

 feldspar varies from reddish to white, and the hornblende is in small grains and does 

 not make up more than one-fifth of the whole rock. Quartz is present in small 

 amount. 



Mic. Under the microscope this rock is seen to be a distinct hornblende granite. 

 Quartz is present in larger quantity than is noticed in the hand specimen. The 

 feldspar is more or less cloudy and many of the grains show a microcline structure 

 and have a wavy extinction, as have also some of the quartz grains. The hornblende 

 is quite fresh and of the ordinary green variety. A few scales of brown biotite are 

 present, and also some green chlorite, which appears as an alteration product from 

 the biotite. Bright brownish sphene is seen in considerable amount, lltucnite, or 

 magnetite, and apatite prisms are also present. One section. 



Age. Archean. . u. s. G. 



No. 499G. GNEISS. 



South shore of small bay on the west side of Snowbank lake, S. E. J S. W. J4 sec. 26, T. 64-9 W. 

 Ref. Annual Report, xx, pages 63, 64, 102. 



Meg. A fine-grained, granular, biotitic gneiss. 



Mic. . In section this gneiss is seen to be a holocrystalline aggregate of inter- 

 locking grains of quartz, feldspar, biotite and hornblende. Many of the grains are 

 elongated somewhat in one direction; this is especially true of the biotite, and there 

 seems to be a tendency for grains of the same size and of the same minerals to be 

 collected somewhat in irregular pai-allel lines. This causes a decidedly schistose 



