102 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
channels through the quartz, either above or below the plane of section. 
In some cases, however, I have noticed these groundmass inclusions sur- 
rounded by the quartz in such a way as to render it probable, at least, that 
they are veritable inclusions of the groundmass dating from the time of 
crystallization of the quartz. In several sections of North Shore porphyries, 
and especially in those of the Great Palisades and of Baptism River point on 
the Minnesota coast, unmistakably genuine inclusions of a true glass are 
to be seen in the porphyritic quartzes. These glass inclusions are in doubly 
terminated ‘negative crystals,” conforming in position exactly with the 
crystal in which they are found, and are of sufficiently large size and 
thickness to test satisfactorily with the polarized light. They show com- 
monly more or less of a trichitic devitrification. Two of these glass 
‘negative crystals” are figured on Plate XIII, at Figs. 6 and 8. 
Porphyritic augites, while far less frequent than the quartzes, are yet 
not very unfrequently to be met with in sections of these rocks. They 
always have crystalline outlines, or remnants of them, being commonly 
more or less deeply eaten into like the other porphyritic ingredients. Their 
chief characteristic is the ferritic decay that they have undergone, the 
whole mass of the crystal being often represented by an opaque, brown, or 
deep-red, or black mass of iron oxide. 
I have already discussed, briefly, the question of the origin of the 
Lake Superior felsites and quartziferous porphyries, and the same question 
is referred to hereafter in other connections. Here it is sufficient to say that 
the marked fluidal structure so often seen, both on the large scale and mic- 
roscopically; the corroded quartzes; the glass inclusions in these quartzes; 
the near approach of the groundmass to the glassy condition; the complete 
identity of these felsitic rocks with others universally conceded to be of 
eruptive origin, and their very close similarity to the undoubtedly eruptive 
rhyolites—all combine to make up an irresistible argument in favor of an 
eruptive origin for these rocks also. 
As typical localities for these rocks—including only places where they 
occur as original masses—may be mentioned the following: (1) for the non- 
porphyritic felsites—Mount Houghton, Keweenaw Point; the central area’ 
of the Porcupine Mountains, and especially the great ledges in Sec. 35, T. 
