Copper-bearing Rocks of Lake Superior. 429 
Grand Island and Marquette the whole series ars around to 
the southwest, on its way to form the western, as it had hitherto 
formed the northern, rim of the great Michigan basin. Where 
this southwesterly bend begins, the outcrop-line of the sandstone 
divides, and from Marquette westward we find, with short in- 
terruptions, the sandstone beds flanking the northern foot of 
the Huron mountains, and dipping gently, 5° to 15°, toward 
the trough of Lake Superior. 
In this part of its course, where it may be said to belong to 
Ripetivk basin proper, it forms a marginal band along 
the lake shore, varying in breadth from a few rods to one or two 
miles. But west of the Huron Islands it widens with the south- 
and often nearly vertical plane of contact, having been seen by 
the earlier geologists at several points along a distance of man 
of faulting motion, was considered as a dislocation. 
able resemblance to those of the eastern horizontal occur, 
apparently conformably overlying the Cupriferous series.* Both 
ee came to be considered as identical in age, and as 
g the upper member of the group. ; 
ere were nan sicenuiniins which made it difficult for 
us to accept this conclusion. One obstacle lay in the enormous 
amount of dislocation required ; for instance, at Portage Lake, 
Where the strata of the Cupriferous series, with an actual thick- 
ru of several miles, dip away from the supposed longitudinal 
ault at an angle of about 60°. 
*It is not yet known the west side of Keweenaw 
Point are upper members "Of the Crpeifctons savice or beloug to the Lower Silurian. 
