324 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
where the base of the group comes to view, there is a steady descent in 
geological horizon. The usual lakeward dip, of course, holds throughout 
the extent of the Temperance River Group. The angle ranges from 6° to 
30°, and on the whole the inclination, especially in the western portion, is 
rather greater than usual. 
In its kinds of rock the Temperance River Group, on the whole, con- 
trasts strongly with the preceding one; indeed, with all of the Minnesota 
shore groups, except the Agate Bay Group, with which it has some charac- 
teristics in common. The rocks forming the greater part of its thickness 
are dark-brownish, fine-grained diabases of the ordinary type, in thin layers, 
with strongly developed vesicular or amygdaloidal upper portions, and often 
with a more or less plainly marked columnar structure in the lower por- 
tions. Much less common, but still occurring in a number of layers, are 
fine-grained, blackish olivine-diabases or melaphyrs, with the typical luster- 
mottlings. They are also furnished with amygdaloids. Layers of ashbed- 
diabase and diabase-porphyrite of conchoidal fracture, and furnished with 
amygdaloids, also-oceur, especially toward the base of the group. Several 
seams of reddish sandstone and shale are included, one layer exceeding 
200 feet in thickness. Peculiar conglomerates also occur. The whole suc- 
cession presents much the appearance of some of the layers in the middle 
and upper portions of the Keweenaw Point series. 
The prevailing diabase and melaphyr of this group do not merit any 
especial description here, since they are only repetitions of what have 
already been described in full for other parts of the extent of the formation. 
They make many very interesting exposures, among which may be mentioned 
as especially fine those of the shore of sections 36 of T.57, R. 7 W., and sections 
30 and 31 of T.57, R.6 W.; that of the mouth of the Manitou River; and that 
of the point on the east side of Pork Bay, where may be seen a black luster- 
mottled olivine-diabase or melaphyr, which, under the microscope, shows 
all the characters of this rock as found in the typical region of Keweenaw 
Point. Another fine exposure is that of the bay in the 8. W. 4, Sec. 21, T. 58, 
R. 5 W., where quite a succession of diabases and amygdaloids is in sight, 
including one or more beds of luster-mottled melaphyr. Those of the 
