42 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
particles as the olivine. It occurs commonly in very irregularly outlined 
forms, much more rarely in elongated parallel rods, in which no distinct 
crystalline outlines can be made out. 
The augitic constituent, which varies from augite to highly fibrous 
diallage, is invariably of later formation than the olivine and plagioclase; 
and commonly, perhaps always, is later than the magnetite also. It often in- 
cludes the olivine grains, and has always its outlines determined by those of 
the previously formed plagioclase. As seen with the naked eye, ona fresh 
fracture, it is of a lustrous black color, more rarely having a tendency to a 
metallic luster. On a weathered surface the diallage is at times of a bril- 
liant, brassy, metallic luster. Itis commonly the coarsest ingredient present, 
its particles at times reaching as much as one or two inches across, even 
when the rest of the rock is not unusually coarse. Very often one crystal 
will present in the thin section a number of wholly detached areas, which 
are proved to be parts of one individual by their common cleavage direc- 
tions, and common behavior between the crossed nicol prisms. 
In some cases, when the augitic constituent is diallage and reaches the 
extraordinarily large sizes above noted, it will include a large number of 
plagioclase crystals, and then the rock presents externally a peculiar appear- 
ance, analogous to the luster-mottling described by Pumpelly as character- 
istic of the Michigan and Wisconsin melaphyrs, only on a far grander scale. 
The body of the rock in these cases is no coarser than usual, nor does the 
hand specimen present any peculiar appearance until it is held in a certain 
position, when a brilliantly flashing, brassy surface is seen, from half an 
inch to two inches across, where before seemed to be only the usual mingled 
ingredients. The explanation of this peculiar appearance, as Pumpelly has 
shown for the melaphyrs, lies in the existence of the diallage in extraordi- 
narily large areas enveloping many plagioclases. This appearance is pre- 
sented in a most striking manner by the rock of the cliffs of the north shore 
of Lake Superior, ten miles northeast of the village of Beaver Bay, Minne- 
sota, and again six miles northeast of the same place, between the Palisades 
and the mouth of Baptism River. The rock seen at these places is of a 
moderately coarse grain and black color. It is very fresh within, but without 
is weathered to a light brown, and on the wave-worn surfaces the eye 
