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



rock, a suggestion leading to the belief that the biotite schists of the dis- 

 trict are secondary after the hornblende rocks. There is not the slightest 

 trace of remnants to demonstrate an earlier condition ; hence strati- 

 graphic relationships and lithologic condition are the data accepted in 

 the interpretation given of the stages of alteration leading up to the ex- 

 isting completed schists. 



The most marked macroscopic feature of these rocks is the uniformly 

 schistose structure. Locall}^ this is varied by the occurrence of more or 

 less contorted veins and sharply lenticular masses of white quartz. Often 

 associated with these are segregations of pink orthoclase, and more 



Figure 3. — Hornblende Schista. 



The original clastic grains have entirely disappeared, and no trace is seen of the original con- 

 dition of the rock. Hornblende quartz and magnetite are the minerals recognized. 1, areas of 

 quartz ; 2, hornblende. 



rarely the dioritic lenses first noted by Kloos * at Little Falls, and since 

 seen at many localities within the district. Ver}^ generally in these more 

 altered phases garnets are abundant. They are especially numerous in 

 those belts containing the lenticular masses of white quartz. At the log 

 dam on Snake river, where the schists are greatly shattered by granite 

 dikes, the texture is coarse. 



Microscopically the quartz is in clear, perfectly transparent grains 

 carrying few impurities, as minute crystals of hornblende and magne- 



* J. H. Kloos : Neues Jahrbuch fur Mineralogie und Petrefacteukunde, 1877, p. 30. 



