344 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
or three times as large and have their longer axes arranged with a tendency 
to a-common direction, while the augite is sparser and in more minute par- 
ticles. The brown color appears to be connected with a ferritic alteration 
of the residuary magma. 
Still another specimen,’ called porphyrite by Macfarlane, and coming 
from the southeast corner of the island, presents an aphanitic, very com- 
pact matrix, of a greenish-gray color, in which are included very abundant 
porphyritic white feldspars, one-sixteenth to one-eighth inch in length, 
and also much rarer and more minute porphyritic black particles. In the 
thin section the base is seen to contain much of a light-brown and dark- 
brown stained non-polarizing material, which is thickly strewn with minute 
tabular plagioclases, and contains also rarer and more minute particles of 
black magnetite. The porphyritic feldspars turn out to be labradorite, the 
erystals of which mineral are often grouped in peculiar clusters, and are 
always very fresh. The porphyritic black particles, seen macroscopically, 
turn out to be augite largely altered to a greenish material, with which there 
is associated much black magnetite, also as an alteration-product. This 
rock, then, is another phase of diabase-porphyrite, with a larger proportion 
of uncrystalline matter. The thin section of this rock is figured on Plate 
IX at Figs. 1 and 2. 
A fourth specimen,’ called by Macfarlane basaltic melaphyr, is aphan- 
itic, nearly black, has a highly conchoidal fracture, with an almost vitre- 
ous aspect, and shows no porphyritic ingredients. It is evidently one of 
the rocks especially referred to by Logan under the name of pitchstone. 
The thin section shows an excessively dense rock, in which, with a high 
power, and with the polarized light, are recognizable very numerous minute 
augite particles, embedded in a non-polarizing matrix, with very much rarer 
minute plagioclases and magnetite particles. This rock is again a diabase- 
porphyrite, but is nearer to the glassy condition than any of those previ- 
ously described. It is also peculiar for its large content of augite. Still 
nearer to the glassy state is a specimen from Sir William Logan’s collection, 
labeled “‘pitchstone.” It is completely aphanitic, of a jet-black color, and 
greasy semi-vitreous luster, and has a glass-like fracture. In the thin sec- 
1 Macfarlane’s No. 16. 2 Macfarlane’s No. 18. 
