362 THE VERMILION IRON-BEARING DISTRICT. 



main medium- grained mass, and penetrate the sediments which surround 

 the granite massive. 



Mineralogically the Snowbank granite varies from a normal mica- and 

 hornblende-granite to an augite-granite, and, by loss of quartz, to a syenite. 

 The hornblende-granites are invariably much darker than the mica-gi'anites. 

 These last tend to reddish colors, while the hornblende-granites are usually 

 dark gray or red if the orthoclase is very prominent and considerably 

 weathered. As the green augite takes the place of the hornblende, these 

 hornblende-granites pass over into the augite-granite. The augite-granite 

 is a grayish, tlesh-colored to red medium-grained granite, and does not 

 differ materially from the normal Snowbank mica- and hornblende-granite in 

 macroscopic appearance. The red augite-granite has been observed to cut 

 the normal Snowbank granite, as for instance on the point projecting north- 

 eastward from the mainland and forming the NW. ^ of the SW. ^ of sec. 36, 

 T. 64 N., R. 9 W. Here the medium-grained red augite-granite cuts a 

 hornblende-syenite phase of the Snowbank granite, and is in its turn cut by 

 a basalt dike. This red augite-granite is also reported to be cut by the 

 hornblende-granite " Both observations are correct, the explanation, as it 

 appears to me, being that they are of essentially the same age and are 

 differentiation products of the same magma. For this reason they are 

 included here together as constituting the Snowbank granite complex. 



No attempt has been made to discriminate on the map between the 

 normal mica-granite, the hornblende-granite, and the augite-granite. They 

 show nothing of peculiar interest under the microscope. The normal 

 combination is mica, quartz, and feldspar, both orthoclase and plagioclase, 

 with some iron oxide in very small quantities. Hornblende is commonly 

 found with the mica, and as it increases in quantity the rock passes to a 

 hornblende-granite. Usually a great deal of sphene is present in the 

 hornblende-granite, and it is more prominent in the hornblende-syenites, 

 which are connected with the hornblende-granites and differ from them 

 only in containing very much less or practically no free quartz. Augite 

 accompanies the hornblende in some of the hornblende-granites, and as it 

 increases in quantity and the hornblende diminishes there is the gradation 

 to the augite-granite. 



aGeol. and Nat. Hist. Survey of Minnesota, Final Eept., Vol. IV, 1899, pp. 427-428. 



