THE ANIMIKIE GROUP. 367 
SrctTion I1.—THE OLDER FORMATIONS. 
THE ANIMIKIE GROUP. 
At Grand Portage Bay, on the east end of the Minnesota coast, there 
rise from beneath the typical Keweenawan diabases, beds of slate and 
quartzite. These beds are finely shown immediately behind the Indian 
village at Grand Portage. Here may be seen a large thickness of a thin- 
laminated, black to dark-gray slate, which is now aphanitic and clay-slate- 
like, and now more distinctly arenaceous. Some of the layers carry nu- 
merous shiny mica scales along the lamination planes, and the whole 
exposure is in striking contrast to anything in the Keweenaw Series above. 
The whole thickness trends N. 70° W. and dips 10° to the 8S. W. The 
cleavage planes are lamination planes and not due to slaty cleavage. A 
great dike, standing vertically, and trending east and west, with a width of 
50 to 75 feet, cuts the slate, which for a long distance is weathered away, 
leaving the dike standing as a bold wall, in places over a hundred feet in 
height. The dike rock is a fine-grained, black diabase, which is peculiar for 
having the black iron oxide constituent in long rods, which often lie parallel 
for considerable distances, two parallel systems at times crossing each other. 
The augite individuals have their contours only in part determined by the 
feldspars. 
At the northeast end of the large island at the mouth of Grand Port- 
age Bay, these slates may be seen directly overlain by the Keweenawan 
diabases, as described on a previous page. The slates do not rise here very 
high above the water, most of the island being composed of the overlying 
diabase. Much of the slaty rock here is a very highly but finely arenace- 
ous, nearly white quartzite, carrying pebbles of white quartz, and consisting, 
as seen under the microscope, of wholly fragmental material, in the shape of 
 subangular to angular quartz grains, mingled with a few of decomposed 
feldspars, all imbedded in a finer material, which appears to be partly 
clayey and partly arenaceous. The dip of these slates, 10° S. E., is in 
entire conformity with that of the overlying diabases. The slates of Port- 
age Bay Island belong above those of the cliff behind the village, having 
their low position by virtue of the southeast dip. 
