100 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
in any of the descriptions of the felsites of other regions that I have exam- 
ined. Only occasionally, as in the pink felsite from the 8S. W. } of Sec. 28, 
T. 56, R. 7 W., on the Minnesota coast (see Figs. 15 and 16, Plate XII1), is 
this networked quartz coarse enough to be readily seen with a low power, 
in the ordinary light. Usually both a high power and the use of the polar- 
ized light are required for its detection, when it appears, in its most charac 
teristic development, as a delicate arborescent tracery or frost-work satu- 
rating the groundmass in all directions. In the polarized light all of the 
quartz network within each of numberless irregularly round areas, whose 
existence would not be suspected in the ordinary light, is found to be simi- 
larly oriented. 
From these more pronounced developments the secondary quartz is 
found through many degrees of lessening amount, and less plainly marked 
character, until it disappears altogether. It is plainly of the same nature 
as the secondary quartz of the already described orthoclase-gabbro, diabase- 
porphyrite, and quartzless porphyry, and of the augite-syenite described 
below. It never, however, reaches in the rocks now under description the 
coarseness, nor presents the graphic form, with which it appears in the 
augite-syenites, its characteristic development here being the delicate arbo- 
rescent clusters above mentioned. Whether this secondary quartz may 
ever be rather a result of devitrification than a truly secondary or alter- 
ation-product I have no means of deciding, though it is certainly the latter 
often, and I should suppose always. It surely can have had no connec- 
tion with the original solidification of the rock. 
The absence in the Lake Superior felsites and felsitic porphyries, so far 
as my observations have extended, of anything like a true spherulitic struc- 
ture, such as is so often met with in rocks of this class from other regions, 
is worthy of note. The occasional radial arrangement of ferrite needles and 
lines of secondary quartz may indicate such a structure, but these appear- 
ances are rare and feebly characterized. 
One or two other unusual occurrences in the thin sections of the matrix 
of these rocks need description. One of these is a faintly greenish, wholly 
isotropic substance, which is present in some of the Beaver Bay porphyries 
in elongated bands and irregular patches. Whether it is to be regarded as 
