STURGEON LAKE REGION 355 



lucent habit. Along the east wall of the valley of erosion which unites 

 with Birch creek, only a few })aces from the first exposure just described, 

 are several other spots where the limestone outcrops, but nowhere is 

 such clear, evenly crystalline material. It is, instead, at the surface a 

 crumbling mixture of quartz and carbonate, so as to be a quartzose lime- 

 stone of quite variable composition. This rock is so easily decomposed, 

 and the beds thereby become so overshadowed, as it were, by the more 

 enduring hornblende schist with which it seems to be interstratified, 

 that the structural relations are difficult to discover. It is quite prob- 

 able that the valley along whose walls these exposures occur has been 

 formed because of the presence of a rock thus easily eroded. 



Several miles to the west of Rutledge, in section 30, township 44, 

 range 21 west, are masses of gneissic rock protruding from the otherwise 

 universal sheet of glacial drift. Coursing through these rocks in various 

 directions and at various angles of inclination are dikes of granite. 

 These dikes are narrow, yet varying in width. Structurally, they are 

 pegmatitic ; mineralogically, they consist of quartz, orthoclase, micro- 

 cline, and plagioclase, with muscovite as the principal bisilicate constitu- 

 ent. The gneissic rocks are modified profoundly by acid intrusives, the 

 most prominent result being a coarser texture, the presence of accessory 

 garnets and pyrite, and the tendency to assume gneissic foliation. 



Snake River Localities 



Along Snake river, in township 42, range 23, through several sections, 

 at intervals a more conspicuous occurrence of granitic dikes and inclos- 

 ing schists is seen. At the log dam and sluice in section 9 occurs a most 

 confused association of these rocks. A diabase dike several feet in width 

 and of vertical position here breaks across the river. The schists are 

 biotite muscovite of a mediumly coarse texture, not only strongly 

 schistose, but even clearly foliated. The muscovite is locally v/ell 

 crystallized and again segregated into nests of radiating individuals 

 pinnately distributed along the fracture })lanes induced in the rock l)y 

 this mineral. 



The granite is usually gray, locally fine grained, but when in contact 

 with the schists inclined to take on a pegmatitic habit having every ap- 

 pearance of coarse-grained granitic dikes. There are many veins and 

 lenticular masses of quartz, some of them reaching a thickness of one 

 foot or more. The jointing of these rocks is quite pronounced. The 

 direction is vertical, and the joints stand from 4 to 10 feet apart. The 

 remarkably zigzag character of the channel, as the river cuts its way 

 through walls nearly vertical and 15 feet high, is apparently due to the 



