286 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
The remaining two-thirds of the Agate Bay Group forms the most of 
the coast from Talmage River to beyond Gooseberry River, in which dis- 
tance there is, on the whole, an ascent of the coast line in geological horizon; 
but there are minor descents and ascents according to the relations between 
the irregularities of the coast and the trends of the strata. The dips are 
flatter than further east, never exceeding 10°, and sometimes sinking to 5° 
for considerable distances. he prevailing rock of the non-amygdaloidal 
portions of the beds is fine-grained and olivine-bearing, having nearly always 
the characters of Pumpelly’s melaphyrs, 7. e., large augites including num- 
bers of minute plagioclases, and much olivine and magnetite crowded into 
the spaces between the augites, the olivine generally altered into a reddish 
or greenish material. Though generally much finer-grained than the typ- 
ical melaphyr of the Greenstone of Keweenaw Point, these rocks often 
show, in the less altered portions, which are then quite black in color, a 
distinct luster-mottling. In less fresh kinds there is a tendency to weather 
to a semi-nodular surface, the augite resisting decomposition better than 
the interspaces. These more altered kinds run through various shades of 
brown and red, more or less mottled with green. There is a good deal of 
variation as to coarseness of grain in different beds, and an extreme coarse- 
ness carries the rock into a true olivine-gabbro, which plainly enough, as 
may be seen even with the naked eye, is a phase of the fine-grained mel- 
aphyr. Such a rock presents itself on a large scale, with bedding surfaces 
hundreds of feet in length and width shelving into the lake at a low angle, 
along the coast between Sucker River Bay and Knife River, in the north- 
east part of T.51, R.12 W. Sections of this rock are figured on Plate III. 
Encampment Island, a mile east of Encampment River, is again formed of 
one of these coarser kinds, which also can be seen forming a distinct layer 
between finer-grained beds, on a cliff side in the north half of Sec. 22, T. 
53, R. 10 W,, 50 to 75 feet above the lake. 
The amygdaloids of the upper two-thirds of the Agate Bay Group are 
very strongly characterized. In the first place they are very highly vesic- 
ular; the vesicles are always small and often so closely crowded that when 
the amygdules are dissolved from them the rock is almost as open as well- 
raised bread. The common amygdules are laumontite, saponite, and cal- 
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