FELSITE AND QUARTZ-PORPHYRY. 99 
many sections, and much of nearly all sections, appears to be made up for 
the most part of microfelsitic and cryptocrystalline matter. Only very 
rarely—as in the light-gray and lilac quartz-porphyries of the islands 
off the north point of Beaver Bay, Minnesota—is there present a non-polar- 
izing material without the red stain. Distinctly recognizable orthoclase 
and quartz particles—as, for example, in a brick-red felsite from the west- 
ern part of the Porcupine Mountains—are not often to be found, though in 
some sections there may be seen floating in the isotropic material, as in a 
cloud, minute tabular crystals of some feldspar. True glass is supposed to 
be present in many sections, interwoven in minute particles with the crypto- 
erystalline and microfelsitic matter, but the determinations made are not 
regarded as satisfactory on account of the imperfections of the instrument 
employed. The large proportion of scaly and granular material which 
produces no effect on the polarized light, and the rarity of true micro- 
crystalline matter, show, however, that we have here to do with a sub- 
stance very close to the original glassy condition, even if it does not con- 
tain true glass. 
The white areas and bands, which in some sections interrupt the prevail- 
ing red background, are always found to be made up of completely crys- 
talline matter, and to include orthoclase and quartz in aggregated irregular 
grains, although it is often difficult, in a given section, to separate these 
materials from one another. The peculiar interlocking of this more crys- 
talline colorless material with the prevailing isotropic, ferrite-bearing, reddish 
base, is plainly and most beautifully seen in many sections to be the result 
of flowage. All of the peculiar and characteristic irregularities which ac- 
company the flowage structure are present here, viz, sudden angular or 
curving bends in the bands, damming against the porphyritic ingredients, 
streams of ferritic particles, brief continuity of individual bands, etc. I 
have attempted to illustrate this fluidal structure in Figs. 3 and 4 of Plate 
XII, and in Figs. 8, 9, 10, 11, and 12 of Plate XIII. 
Although wholly absent from some sections, a very highly characteristic 
feature of the sections of many of these rocks, and more particularly of the 
felsites without porphyritic quartz, is a networked quartz which can only 
be regarded as of secondary origin. I find no mention of such a feature 
