166 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
ern or northern lowland is again underlain by a sandstone, which in this 
case, however, is the highest member of the same series that gives rise to 
the central ridge, whence the merging into one another of this ridge and 
the western lowland. In the Portage Lake region and westward from that 
vicinity there is one broad central ridge rather than a series of parallel ridges, 
on account of the comparatively high dip of the strata—38° to 65°. The 
western lowland only begins in the vicinity of Gratiot River, because 
further east the upper sandstones lie beneath the lake. The eastern part 
of Keweenaw Point is marked by a series of parallel ridges with cliffy 
southern and flat northern faces, because here the rocks lie flatter, the softer 
amygdaloids and more easily decomposable diabases wearing into valleys, 
while the more resistant melaphyrs, coarse diabases and bowlder-conglom- 
erates are left in relief. 
Many other connections between the topography and geological struc- 
ture of this region might be shown to exist. Not the least interesting of 
these is the relation of the peculiar line of long narrow lakes and bays, 
which flank the northern side of Keweenaw Point in its eastern extension, 
to the easily eroded amygdaloids and other soft beds, out of which these 
bays are worn. Each bay has a sea-wall of some of the more resistant 
layers. 
In the eastern part of Keweenaw Point, east of Eagle River, there is 
a maximum thickness of rock of about 25,000 feet below the base of the 
outer conglomerate, which horizon I have already selected’ as the dividing 
line between the Upper and Lower Divisions of the series. This line in the 
eastern part of the point lies near its northern margin, the dip here being 
northward, and the strata trending with the curving course of the point 
itself. As already indicated, neither the upper nor the lower limit of the 
series is reached on Keweenaw Point, the downward extension being buried 
beneath the newer horizontal sandstone of the southern lowland, while the 
upper limit lies under the waters of the lake. Moreover, the southern limit of 
the exposures on Keweenaw Point is, as explained subsequently, a fault line, 
running the entire length of the point, and westward beyond the Ontonagon 
River. This fault does not follow the strike exactly, but cuts across it 
‘Chapter V, p. 152. 
