306 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
far. The brownish alteration-product often gives a resinous look to the 
hand specimen. 
On Beaver River this black rock in one or two places shows indications 
of a southeasterly dip of some 18° to 20°, but on the north point of Beaver 
Bay, where it is to be seen in bold ledges, with a finely developed columnar 
structure, the dip is flatter. 
At several points along Beaver River this black gabbro carries large 
masses of an anorthite-rock similar to that described as occurring further up 
the coast. The bowlder-like character of the anorthite-rock, though often 
pronounced, is not always so plain, and in some places it looks more like a 
dependency of the prevailing black gabbro. Thin sections of this rock 
occasionally show rare particles of augite or diallage between the feldspar 
grains. 
The acid rocks of this vicinity are found forming the south point of 
Beaver Bay, along the shore of the bay at the points D E F, where they 
are confusedly intermingled with the basic rocks, on the small island on 
the north side of the bay, and ‘on Cedar Island, which lies some 500 yards 
east of the north point of the bay. Both of these islands are composed of 
quartzose porphyry. 
The bold point on the south side of Beaver Bay is a mass of pink 
granite-like rock, bounded on all sides by precipices about 100 feet high, 
and rising in the center to an elevation of 130 feet above the lake. The 
rock of which this point is composed has a pale flesh color, and presents to 
the naked eye the appearance of a highly feldspathic granite of medium to 
fine grain. Shining feldspar facets appear to make up most of the rock. 
The thin sections seem also to be almost entirely made up of feldspars, but 
always much altered and changed, with much secondary quartz. In some 
sections where this replacement has been carried far, it is impossible to tell 
whether we may not have also some replaced matrix. Black and opaque 
particles of magnetite, ferrite, and augite are the accessories. Some of the 
clusters of black particles appear as if due to the alteration of augite erys- 
tals. At the point C this red rock is seen surrounded on three sides by a 
mass of rather fine-grained diabase, in which the anorthite crystals are often 
inclosed in numbers by single augites. 
