ROCKS OF BEAVER BAY. 305 
bed of Beaver River, and on the south point of Beaver Bay, is the same 
as that already described as showing for some miles along the coast above 
Beaver Bay. It is a moderately coarse, very dark-colored, and often nearly 
black, very highly crystalline rock, in which the three common ingredients— 
augite, anorthite, and titaniferous magnetite—can often be seen with the 
naked eye. The very dark color is due to the great abundance of the 
augitic ingredient, which, while commonly subordinate to the plagioclase 
x 
g 
% 
Sip 
~ 
: 
Ushbed- diibase and granitic porphyry. 
LAH Orthoclase= (boro weitle Tine - 3 
A | olivine a ro and telsite 
Y <I Anorthite Rok 
Scalel400 feet tol inch. 
Fig. 18.—Sketch-map of rock exposures in the vicinity of Beaver Bay, Minnesota coast, T. 55, R. 8 E. 
in this class of rocks, here greatly predominates. It shows all gradations 
between augite without foliation, and very highly foliated diallage. It is 
commonly quite fresh, but is also found more or less completely altered to 
a greenish material. It is not only the most abundant ingredient, but is 
also much the coarsest. Olivine is absent from some sections, and is never 
abundant, but when present is seen in very large areas, in which an alter- 
ation to a reddish-brown ocherous substance has always progressed very 
20LS 
