356 C. W. HALL — KEEWATIN OF EASTERN AND CENTRAL MINNESOTA 



strength of this jointing and the success of the river in following these 

 lines of least resistance as it carved its bed in the fresh crystalline rocks. 



Rum River Valley 



Along the Ann and upper Rum rivers, including some hitherto unde- 

 scribed localities around the east side of lake Mille Lacs, are many ex- 

 posures of a medium grained hornblende biotite granite with no visible 

 areas of schists. There are two types of granitic rocks : one red and 

 somewhat coarse grained, seen along the west branch of the Rum river 

 and its tributaries ; and the other light gray and medium grained, char- 

 acteristic of the Ann river valley. There,are representatives of the two 

 granite t3'pes, the red and the light gray, found in the region interven- 

 ing between the Kettle river on the east and the Mississippi on the 

 west, comprising several thousand square miles in the very center of the 

 state. These Rum River and Snake River outcrops would seem to be 

 along the eastern border of the granitic area, where the exposures dis- 

 close a series of dikes breaking into and markedly modifying the older 

 schists which stretch south westward from the graywacke beds of the 

 Thomson district. 



Morrison County 



Northwestward from the foregoing, between lake Mille Lacs and the 

 Mississippi river, at the mouth of the Elk, are many exposures of granites 

 and gneisses. At the Mississippi itself are extensive areas of hornblende- 

 biotite schists carrying garnets and staurolite in profusion. At the large 

 reef below the mouth of the tril)utary Swan river a bed of rather fine 

 grained pink limestone was reported several years ago. The accounts 

 would seem to make it identical with the limestone exposed southwest 

 of Sturgeon lake. If this surmise be correct, the two localities have be- 

 tween them many intrusions of granitic eruptives. At Little Falls and 

 westward are intrusions of basic rocks, both diabase and gabbro. Ex- 

 tensive exposures of biotitic olivine gabbro occur in section 13, township 

 129, range 30 west, and in a succession of outcrops southward far into 

 Stearns county. In the northwestern corner of Morrison county altered 

 schists occur striking westward and standing nearl}^ vertical. In their 

 alteration much calcium carbonate has been formed, which at the pres- 

 ent time constitutes a considerable bulk-percentage of the schists. Be- 

 3^ond Morrison county northwestward the schists have not been seen. 

 Eruptives, both acidic and basic, are exposed ; the former as epidote 

 granites at Ashle3% Ward, and in southwestern Cass county ; the latter, 

 besides several diabase dikes at several localities, in an area of interest- 

 ing apatite-diorite at the mouth of Fish Trap brook. Northward of the 



