322 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
crystals, marked in places by a network of black lines. Under the micro- 
scope this rock turns out to be composed entirely of crystalline matter; 
reddened oligoclases and orthoclases, completely saturated with secondary 
quartz, making up most of the section. The black markings mentioned 
resolve themselves into altered augite blades, with whose alteration is con- 
nected the production of much magnetite. Numerous large apatite needles 
are included. 
Below the mouth of the river forming the western boundary of the 
Indian Reservation, coarse olivine-gabbro forms the coast as far as Red 
Rock Bay, a distance of some five miles, lying in beds with cross-columnar 
structure, flat lakeward dip, and easterly trend. In many places this rock 
shows in a marked manner the luster-mottling due to the presence of rela- 
tively large diallages, including many feldspars. 
At Red Rock Bay, in the Indian Reservation (southeast part of T. 63, 
R. 5 E.), these black rocks give place again to red felsite and quartziferous 
porphyry. The display of these acid red rocks here is very large and 
among the most interesting of this class of rocks in the Lake Superior region. 
The red rock at the bay is commonly atrue quartziferous porphyry, with a 
base in which much non-polarizing matter is mingled with the usual ferrite 
particles, and saturated with arborescent secondary quartz. On the high 
point of red rock in the bay, areas of a slightly differently colored rock are 
included in the general red mass. Under the microscope the rock of these 
areas shows a base in which quartz and orthoclase are distinctly individu- 
alized in good-sized particles, The bold red bluff from which the bay takes 
its name shows no trace of banding, but at the point beyond the bay the 
curving and twisting pseudo-lamination so often seen in these red porphy- 
ries is shown on a large scale. For short distances the rock will appear like 
a much-contorted schist, and then again will pass into the general structure- 
less mass. When seen, the banding is as often vertical as horizontal. 
In this vicinity the red rock is cut by a heavy dike, of which part is 
represented in the accompanying figure. This dike shows for several 
hundred feet along its length, and with a width of some 75 feet. The 
junctions with the adjoining rock are sharp, occasionally showing irreg- 
ularities, as in the figure. In places patches of red rock clinging to the top 
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