TERTIARY VOLCANIC ROCKS-LATITE-PHONOLITE. 
77 
which exhibits aggregate fibrous polarization and can with certainty be called 
chalcedony, lliese three minerals thus form a somewhat complex decomposition 
pseudomorph after titanite. (See PI. XVII, E.) The alteration appears to take 
place with equal readiness away from and near mineral veins. 
Other minerals .—Magnetite occurs rather abundantly, particularly in those 
varieties holding numerous small grains of pyroxene. There is in this fact a pos¬ 
sible significance that the two minerals have resulted from the magmatic resorption 
of hornblende, but it seemed impossible to reach any conclusions as to this matter. 
The mineral occurs especially near crystals of apatite and is usually of later forma¬ 
tion. Oxidation changes it more or less completely to hematite and often it 
furnishes on decomposition an opaque, nearly white, material whose identity could 
not always be established, but which is in some cases certainly a carbonate. Grains 
of partially decomposed black iron ore are sometimes surrounded by minute 
particles of titanite, indicating that part of the iron ore is titaniferous. 
Small brownish crystals of zircon occur in a few varieties. 
In one or two rock masses, particularly in the biotite trachyte on the western 
slope of Mineral Hill, small interstitial grains of primary quartz are evenly dis¬ 
tributed through the groundmass. 
The green secondary mica mentioned as a product of the decomposition of 
pyroxene, hornblende, and biotite does not seem to correspond to any of the varieties 
described in the text-books. It varies from deep grass-green through light and 
greenish yellows to strong yellow or yellowish brown, but is always distinctly 
separable by its color from ordinary biotite. It occurs uniformly as aggregates of 
small fragments, shreds, or fibers which sometimes have an imperfect radial 
arrangement. It is analogous to sericite in texture, but somewhat coarser grained. 
Pleochroism is noticeable in the deeper-colored masses. The polarization colors 
are sometimes rather lower than for either biotite or muscovite, but frequently 
are very brilliant and always have that peculiar character distinctive of the micas. 
Either it has a variable composition or it is built up by the addition or subtraction 
of substance in the minerals from which it forms, for it completely and pseudo- 
morphically replaces pyroxene, hornblende, and biotite, and it is almost certain 
that those minerals do not have identical composition. This change is not limited 
to the vicinity of mineral veins where metasomatism is likely to take place, and it 
is consequently probable that the mineral is variable in composition. 
Near mineral-bearing veins fluorite, carbonates, pyrite, and sometimes a little 
quartz and chalcedony may be introduced into the rock, either as isolated grains or 
as veinlets. In many cases, pyrite partially or completely replaces magnetite, and 
on that account a rock with considerable unaltered magnetite may usually be 
safely assumed to be at some distance from a vein. 
TRACIIYTIC FACIES. 
As has been previously stated (p. 71), a rock which occurs on the northern 
slope of Battle Mountain and northwest of the summit of Mineral Hill differs in 
appearance from the ordinary latite-phonolite by containing numerous small flakes 
of dark-brown biotite. It is the same rock that was called augite-mica andestite by 
13001—No. 54—06-7 
