154 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
east side of Keweenaw Point;' but it is to be observed that it is not 
structurally impossible that these horizontal sandstones should be the same 
as those I have been considering, since, according to the structure I have 
worked out for this part of the basin, the upper Keweenawan sandstones 
should themselves have here a position not appreciably different from 
horizontality? 
The upper sandstones appear to view again on the south side of Isle 
Royale, with a considerable development. Caribou Island, south of Mich- 
ipicoten, appears also to be made of them, but unless the sandstones east 
of Keweenaw Point represent their upward extension, a point which is 
subsequently discussed, and which I cannot doubt should be decided in the 
negative, they do not appear to view at any other point. Still they must 
have a very extensive development underneath the waters of Lake Su- 
perior in its eastern as well as in its western portions. 
The Lower Division of the series must, from the nature of the case, 
present very considerable variations in subordinate stratigraphy and total 
thickness. For portions of the basin, and even over areas 50 to 200 miles 
in length, it is possible to recognize a pretty constant subordinate arrange- 
ment, the constancy increasing, of course, in inverse ratio with the size of 
the district considered. It is not possible, however, to lay down any 
scheme of subordinate stratigraphy which shall hold for the entire extent 
of the series. This results not only from the mode of formation of the 
rocks, but from the great similarity of the beds at different horizons, and 
through great thicknesses. Nevertheless it is possible to make a number of 
generalizations as to the characteristics of broad horizons which will hold 
throughout most of the geographical extent of the Lower Division of the 
series. These are enumerated in the next paragraph. 
(1) Coarse-grained rocks, including both orthoclase-free and ortho- 
clase-bearing kinds, though occurring now and then well up in the Lower 
Division (e. g., some of the beds of the Greenstone Group of Keweenaw 
Point), are very much more common at low horizons, and the very coars- 
1Geology of Wisconsin, Vol. III, Part I. 
*See structural map, Plate XVIII, and the explanations of Chap. IX. 
