382 COPPER-BBARING ROCKS OF LAKE SUPERIOR. 
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larity of the crystalline rocks found capping hills in different parts of this 
region. Not only is it much more in accordance with the geology of the 
entire Lake Superior region to suppose these occurrences to represent many 
different flows, but there is distinct evidence that they do so in many cases. 
This evidence consists in part in actual visible interstratification with the 
slates, in some places, of great beds of olivinitic gabbro, identical both 
macroscopically and microscopically with the rock capping Thunder Cape. 
Another evidence is the very great irregularity of level that this supposed 
flow must occupy, the height at which it is found varying back and forth 
through distances of several hundred feet. A yet stronger evidence is 
found in the general structural character of the region, by virtue of which 
each heavy, enduring, crystalline rock layer constitutes a ridge with a long 
front slope and a precipitous back slope. This structure is especially well 
marked in the region of the Pigeon River, Minnesota.’ Even a general 
distant view of the belts to the west and south from the head of Thunder 
Bay is strongly suggestive of this structure. . 
I have no confidence, then, in the existence of any one crowning over- 
flow, as supposed by Logan, Bell, and Macfarlane. Bell goes so far as to 
identify all exposures of rock, lithologically similar (macroscopically) to 
the Thunder Cape gabbro as parts of the crowning overflow, even as far 
north as the region about Lake Nipigon. Lithological similarity is no 
evidence of original continuity in this case, for all through the Keweenaw 
Series of the North and South Shores, similar olivinitie gabbros are visibly 
interstratified at all sorts of horizons. 
As already indicated, the Animikie slates have been traced along the 
national boundary as far as Gunflint Lake by Bell,’ and more recently by 
N. H. Winchell. Thence their northern boundary extends southwest from 
Gunflint Lake. For many miles farther southwest the region traversed by 
these rocks has not been examined by any geologist, but their continuance 
here cannot be doubted, for in the Mesabi Range, in T. 60, R. 13 W., 
Minnesota, they show in full force. By the kindness of Prof. A. H. 
1N. H. Winchell, in Ninth Annual Report of the Geological and Natural History Survey of Min- 
nesota. Minneapolis, 1881. p. 76. 
3 Report of the Geological Survey of Canada, for 1872~’73, pp. 92-94. 
‘Ninth Annual Report of the Geological and Natural History Survey of Minnesota, pp. 82, 83. 
