348 C. W. HALL KEEWATIN OF EASTERN AND CENTRAL "MINNESOTA 



alternations of impure quartzite, so indurated that the coarser pebbles 

 are fused with the finer material into a semi-cr^^stalline mass and a typ- 

 ical argillaceous slate with prominent cleavage. 



Irving^ thus enumerates the rock species: 



"A mono; the slates, fine grained graywacke slates, clay slates, sericitic quartz 

 slates, true qiiartzites, mica slates (often hornblendic), staurolitic mica slates (often 

 garnetiferous), and hornblende schists, and among the eruptives, diabases, gab- 

 bros, and diorites, the latter presumably altered forms of diabase or gabbro." 



The rocks have been traced southwestward for many miles and found 

 to merge into a series of well defined hornblende-mica schists. A total 

 obliteration of clastic cliaracters and an interbedding of carbonate bands 

 is shown. Still farther southwest a distinct limestone formation enters 

 the series and becomes geneticall}'' an important factor. 



STRUCTURAL FEATURES 



The strike avera,ges south 70 degrees west, and the dip south at an 

 angle varying between IC and 60 degrees. The slaty cleavage, which 

 usually can be distinguished from lamination, is nearl}^ vertical, with a 

 direction nearly east and west. 



At Thomson in both theslat}^ bands and the schists are locally numer- 

 ous carbonate concretions. 



The slaty cleavage seems to characterize the layers of finer and more 

 argillaceous material, alternating with the quartzitic and conglomeratic 

 phases. It always ceases at the contact of the argillaceous and silicious 

 layers. The angle which it makes with the plane of contact, assumed 

 everywhere as the bedding plane, varies, since the bedding of the rocks 

 several times across the formation changes its attitude with the horizon. 

 The quartzite shows the effects of enormous pressure, yet the ph3^sical 

 condition of the rock material, so much less plastic, was so resistant that 

 the eff'ect of slaty cleavage was not induced. 



This alternation of quartzitic and slaty layers is a notable petrographic 

 character along the Saint Louis river. It is distinctly seen through color 

 differences, lithologic characters, and rapidity of weathering. The coarser 

 material carries zones or planes of nodular concretions which are of a 

 decidedly carbonated composition. Weathering brings out the position 

 of these concretions rapidly and in a most conspicuous manner. They 

 indicate bands of the sedimentar}'' rocks which contained a much greater 

 per cent of carbon dioxide in combination. Through the subsequent 



* R. D. Irving : Fifth Ann. Kept. U. S. Geol. Survey, 1885. p. 197. 



