YE COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
where, they often form parts of the same rock, the groundmasses of the 
two rocks being identical in constitution. 
These rocks belong to a class which is, as is well ‘ca one of the most 
difficult of all the rock groups to study with the microscope satisfactorily. 
Very widely separated views have been held as to the nature of their 
aphanitic matrix, even since the use of the microscope in their study. The 
differences of opinion have been chiefly as to whether the matrix is crys- 
talline, partly crystalline, or wholly uncrystalline. In his discussion of this 
subject, Rosenbusch has shown that both completely crystalline and partly 
glassy matrices occur, with many intermediate stages, and that uncrystalline 
and completely crystalline material often occur intermingled in the same rock. 
Calling all parts of the matrix which are doubly refracting, crystalline—so 
long as it cannot be shown that the double refraction is a result of mechan- 
ical strain—Rosenbusch designates as microcrystalline those porphyritic 
groundmasses, or parts of groundmasses, which are made up of mineral- 
ogically determinable particles; as cryptocrystalline those which are ag- 
gregates of mineralogically indeterminable, but still doubly retracting 
particles, and as glass or glass basis, those which show no polarization effects 
whatever, even when examined under the highest powers. In place of the 
true glass basis, there is often to be seen interwoven with the microcrys- 
talline and cryptocrystalline material, a completely isotropic substance, 
which is colorless, grayish, yellowish or brownish, and which differs from 
the true glass, in that it is not wholly structureless, but is, on the con- 
trary, made up of extremely minute scales, fibers, granules or aggregates 
of granules, and other grouped forms. This substance, “which is distin- 
guished from the microcrystalline and cryptocrystalline aggregates by lack 
of action upon the polarized light, and, on the other hand, from the true 
glass by lack of structurelessness”’—Rosenbusch calls microfelsite or micro- 
felsitic base. In the following descriptions, the Rosenbusch nomenclature is 
followed. 
The matrix of the Lake Superior quartz-porphyries and felsites, as seen 
macroscopically, presents usually some quite marked shade of red; the 
‘Conf. H. Rosenbusch, Microscopische Physiographie der Mineralien und Gesteine. Band II, pp. 
51-76. 
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