294 THE VERMILION IRON-BEARING DISTRICT. 



slates vary in thickness from a fraction of an inch to several feet. The 

 bands of slate themselves, where interlaminated with grits and near the 

 conglomerates, also vary in thickness from a few inches up to 30 paces. 

 These slate beds show a gradual increase in thickness as they occur at a 

 greater distance from the conglomerates. The slates are in places — as in 

 the embayment between Tower and Lee hills — very heavily impregnated 

 with pyrite, which is scattered through them in cubes, usually altered 

 more or less to limonite. 



Microscopic examination of the Knife Lake slates and associated gray- 

 wackes shows that the primary constituents are feldspar, quartz, and horn- 

 blende in fragments. With these occur secondary products — chlorite, 

 epidote, calcite, sericite, sphene, and pyrite. In the coarse-grained rocks 

 the various constituents can readily be distinguished. In the finer-grained 

 ones only the coarser particles can be clearly recognized, and these lie in a 

 very fine-grained dark matrix whose characters can not be positively 

 determined, but which probably consists of fine dust particles derived 

 from the other constituents, with which may occasionally be associated 

 some carbonaceous material (although this was not recognized as such) 

 and ferruginous matter, the last being the chief cause of the dark color. 



These slates vary from the normal slates described, which prepon- 

 derate, to rocks found in certain portions of this area, which, although 

 showing all the macroscopic features of bedded elastics, nevertheless under 

 the microscope are seen to have been recrystallized, and now may properly 

 be called mica- and amphibole-schists and gneisses. These mica- and 

 amphibole-schists and gneisses vary from light-gray to nearly black rocks. 

 The schists have a light-brownish weathering crust. One can distinguish 

 in all cases in them the mica flakes, the amphibole, and very frequently 

 the feldspar and quartz. Difierences in color and size of grain produce a 

 banding in these metamorpliosed rocks. Usually the banding stands out 

 much more plainly on their weathered surfaces than upon the fresh fracture 

 planes. This banding in the schists unquestionably corresponds to lines of 

 original bedding, for it can in places be traced uninterruptedly from the 

 slates into the banded mica-schists, in both of which rocks it shows the same 

 strike. Moreover, at one place south of Tower, on exposures east of the 

 Duluth and Iron Range Railroad, near milepost 92, one may see these 

 schists in various stages of formation, and on these schistose rocks there 



