﻿Iron Ores of the Lake Superior region. 269 



region are yet more strikingly displayed here, for here 

 the regularly banded and contorted jasper may be seen 

 traversed, not only by tongues of ore, but by seams of jasper 

 and of jasper and ore together in every conceivable shape. 

 Unfortunately for the theory, however, the same exposures 

 give, in the gradation of the whole rock mass into manifestly 

 sedimentary slates and in the manifest community of origin 

 between the white purely quartzose silica and the jaspery ma- 

 terial, conclusive arguments against any such hypothesis. In 

 places where the folding has been most violent the rock is 

 often shattered, as it is seen to be in the Marquette country, 

 but to a far more violent and striking degree, and in these 

 shattered portions the cementing material may be ore, or jasper, 

 or ore and jasper together, or the whiter chalcedonic silica. It 

 should not, however, be understood that these brecciated and 

 veined portions form any very large proportion of the whole 

 exposure. In the main it shows the regularly laminated 

 though often highly contorted jasper. It should be said, in 

 connection with the matter of the gradation above said to obtain 

 between the jaspery schists and the gray carbonaceous slates, 

 to avoid possible misunderstanding, that there are here also, 

 interbanded with the ore belts, light-colored schists of a totally 

 different character, between which and the ferruginous mate- 

 rials there are no gradations. 



If no microscopic work had been done and if no evidence to 

 the same effect had been collected in other portions of the Lake 

 Superior region, these exposures alone would be sufficient, I 

 think, to establish the formation of the jasper-schists from some 

 sort of sedimentary slaty rocks by a process of silicification. 

 This conclusion is borne out very thoroughly by the appear- 

 ances of the thin sections, which in addition furnish us with 

 quite striking evidence of the nature of the original material. 

 The darker-colored slates are found in their least altered con- 

 ditions to contain a considerable quantity of iron carbonate. 

 This presents itself, not merely in detached individuals in the 

 ground-mass of finely divided interlocking silica, but also in 

 areas of considerable size. These areas are aggregates of the 

 crystalline or rhombohedral carbonate, and are most manifestly 

 the oldest portions of the. rock. The larger patches, in which, 

 it should be said, the carbonate is at times more or less mingled 

 with carbonaceous material, have evidently been separated 

 from one another by the interlacing seams of silica ; while from 

 them to single rhombohedra, lying isolated in a ground mass 

 of silica, the carbonate is present in pieces of every size. At 

 times this rhombohedral carbonate has been changed directly 

 into sesquioxide of iron ; in other cases, however, it is mingled 

 with actinolite, magnetite and silica in such manner as to 



