AO COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
In the table given below the angular measurements always represent 
the largest angles that could be found. It will be noted at once that two- 
thirds of these measurements are high enough for anorthite—the most basic 
of the plagioclases—while in all the remaining sections entered as contain- 
ing labradorite, angles were obtained not far below the upper limit for that 
mineral. In some of these cases there was difficulty in finding enough 
plagioclase sections in the right zone, and other slices might have given 
higher angles. 
The plagioclastic ingredient of these rocks is commonly quite fresh, 
and always fresher than the olivine or augite. When alteration occurs, it is 
usually but a slight cloudiness. In the angular measurements made on 
over two hundred sections of the several kinds of basic rocks, it was ob- 
served that the freshness of the plagioclase bears a distinct relation to the 
size of the angle between the maximum-extinction positions of the adjacent 
hemitropic bands, the freshest feldspar always giving the largest angles, 
and vice versa. Pumpelly gives an instance (that of the rocks of bed 96 of 
the Eagle River section, Keweenaw Point) of a change of the plagioclase 
to prehnite in a rock of this class. This is an alteration commonly noted 
in the finer diabase, but in the rocks of the class now under description it is 
well-nigh unknown. 
In sections of the very coarse-grained gabbro of Bad River, Wisconsin, 
large areas are often seen which polarize monochromatically, and are hence 
parallel to the brachy-pinacoid. These areas are thickly crowded with 
minute black needles, arranged in several directions. The needles, whose 
nature is in doubt, are characteristic’ of the gabbros of many other regions. 
In the Bad River rock the set of the needles lying parallel to the vertical 
axis includes very much the larger number. Other needles, making an 
angle of 112° with the first, and evidently placed parallel to the strong 
basal cleavage, are fewer in number, but much longer. Still others, very 
numerous, lie oblique to the plane of the section, and are parallel to pyra- 
midal planes.? The occurrence of the large monochromatically polarizing 
1Geology of New Hampshire, Vol. III, Part IV, pp. 94. 
2Compare Geology of New Hampshire, Vol. Ill, Part IV, Plate V, Fig.5; also Fig.3, Plate XV D, 
Geology of Wisconsin, Vol. III; also Fig. 58, Plate X, Rosenbusch’s ‘‘Microscopische Physiographie 
der massigen Gesteine.” 
