330 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
ceptible on the Minnesota coast before reaching Grand Portage, and shows 
also in the Lucille Islands, off Pigeon Point, the outer one of these islands 
being formed of a typical Keweenawan diabase. 
The following brief account, with the accompanying map of Plate 
XXVII, will serve to present the main features of the geology of this 
region. So far as Isle Royale is concerned, I have had to depend upon the 
report of Messrs. Foster and Whitney, read in the light of a familiarity with 
most of the remainder of the extent of the formation, not having visited the 
island myself. The detailed topographical map of the island, by the United 
States Lake Survey, aids not a little in the understanding of the structure. 
The region from Thunder Bay to Nipigon Bay I have examined myself, 
and am thus able to draw information more satisfactorily from the descrip- 
tions of Logan, Macfarlane, and Bell, as well as to judge of the correctness 
of the views advanced by these writers. 
ISLE ROYALE. 
Isle Royale is a very long, narrow island, trending in a general north- 
easterly direction. Irom point to point the island is just 45 miles in length; 
but from the Rock of Ages, the farthest outlying reef to the southwest, to 
the Gull Island rocks on the northeast, is 57 miles. The island varies in 
width from three to eight miles. It does not lie exactly in a straight line, 
but curves from N. 65° E. in the southwest part to N. 53° E. in the north- 
east part. On the southern side of the southwestern end is quite an area of 
low land, underlain by sandstone and conglomerate, dipping some 8° to the 
southeast. This sandstone evidently belongs to the Upper Division of the 
series. ‘The remainder of the island is made up of very regularly bedded 
crystalline rocks, with here and there an interstratified conglomerate, all 
dipping southward at an angle which has not been satisfactorily determined, 
but which, probably, does not often exceed 25°. 
The whole shape of the island, both as to outline and topography, 
expresses the geological structure in a most striking manner. It is traversed 
from end to end by a series of parallel ridges, which present always a steep, 
often a precipitous side towards the north, and a gradual slope towards the 
‘Since the above was written, N. H. Winchell has published some notes on the geology of Isle 
Royale, especially on the south side of the island. Tenth Annual Report of the Geological and Natural 
History Survey of Minnesota, pp. 49-54. 
