1895. | NEW YORK ACADEMY OF SCIENCES. — 197 
FELSITE-PORPHYRY. Pl. XII., XIII., and Fig. A p. 199. 
Under this group may be placed the majority of the effusives 
of the Huronian. Most of the “ felsites ” and “ petrosilex ” of 
the Canadian Survey Reports are either porphyry or porphyry- 
ash ; and some of the rocks described as sandstone, etc., prove 
on the evidence of thin sections to belong here. The central 
and most abundant type is a quartz-free porphyry with scat- 
tered phenocrysts of orthoclase and plagioclase in a micro- 
granitic or microfelsitic groundmass. Quartz phenocrysts occur 
in a few sections; in others the amount of twinned feldspar in- 
creases relatively to the untwinned till the rock is, strictly 
speaking, a porphyrite. Flow structure is seen in most of the 
sections, and trichites, spherulites, perlitic cracks and other 
characteristically volcanic structures have been observed. Brec- 
cias are abundantly found, sometimes very coarse, the fragments 
being six inches to two feet in diameter. Finer grained rocks, 
sometimes distinguishable as composed of sharp-edged angular 
fragments, more often not determinable, are still more common. 
From the absence of any accompanying rocks of distinctly sedi- 
mentary character it is perhaps safest to place these as in most 
cases fine ashes or tufts. 
This group of rocks bears every indication of being of strictly 
superficial origin. Their texture is more or less irregular; they 
are frequently vesicular and flow-brecciated, with few. scattered 
and often broken phenocrysts, being contrasted in these char- 
acters with the compact, smooth and uniform appearance and 
abundance of phenocrysts seen in the last group. 
Quartz phenocrysts occur quite rarely, are often broken, but 
seldom notably resorbed. The quartz in the groundmass is more 
important. In perbaps a third of the sections examined it 
seems to be an essential constituent, distinguishable from the 
feldspars by its brighter polarization colors; it is granular and 
rarely at all intergrown in granophyric forms. In many cases 
it is certainly a devitrification product,as isshown by the rem- 
nants of original glassy structures still traceable. 
Orthoclase phenocrysts are found sparsely scattered in all the 
sections. They are never very large, mostly 1-4 mm. in length, 
are rarely resorbed, but not uncommonly broken and the frag- 
ments displaced. They are usually twinned after the Carlsbad 
law, in one case after the Baveno. The orthoclase in the ground- 
mass is mostly granular, but part of the rod-like feldspar in the 
groundmass of some sections may be monoclinic. 
Plagioclase is in some sections more abundant than orthoclase, 
and rarely entirely fails. Its crystals are less regular in outline 
