THICKNESS OF BEDS. 141 
much longer stretches. The belt of thin amygdaloids and diabases lying 
above the great conglomerate of Eagle River, Keweenaw Point, with a 
thickness of some 1,500 feet, must run uninterruptedly from the eastern 
extremity of Keweenaw Point to the Wisconsin boundary, a distance of 
150 miles. Several of the groups into which I have divided the rocks of 
the Minnesota coast can also be recognized for many miles, and there is a 
strong probability that one or two of them exist on Isle Royale. 
The thickness of the individual beds of these basie rocks has been 
already given as ranging from ten or under to several hundred feet, but 
for the greater portion of the series the thickness is less than 100 feet. 
Towards the base of the series, among the older flows, however, beds occur 
of great thickness, and present massive exposures in which it is very hard 
to see any sign of structure. At this horizon in the Bad River region of 
Wisconsin are immense structureless masses of a very coarse-grained gray 
gabbro, and similar rocks have a great development at a similar hori- 
zon in Minnesota. Fine-grained diabases also occur largely at these low 
horizons without any such distinct bedding as is characteristic of most of 
the series; for instance, on the so-called “South Copper Range” of Mich- 
igan, between the Montreal and Ontonagon rivers, and in the Duluth 
Group of the Minnesota coast. The apparent lack of bedding in the 
latter region is doubtless due to the much greater thicknesses of the in- 
dividual beds, and to their nearly vertical position, but it is possible 
that the coarse-grained rocks of the Bad River and Duluth regions may 
owe their want of structure to a different cause, as is subsequently indicated. 
The effect of the prevalent bedded basic rocks upon the topography is 
everywhere very marked, having a common character, varied only by the 
varying dips and varying thicknesses of the individual layers. Since the 
dip is almost always lakeward, the common effect is a longer lakeward or 
front slope, and a steep or precipitous back slope. Where the dip is flat, as 
all along the Minnesota shore, the front slope coincides with the dip slope, 
and the shore line of hills ascends at an angle of from 5° to 10°, to drop off 
suddenly in the rear. The valleys of the streams entering the lake nearly 
at right angles divide these hills into detached blocks. Such a series of 
blocks is well seen as one looks towards the west from Grand Marais. 
