PART II.
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HYPERSTHENE-DACITES.
DISTRIBUTION AND DESCRIPTION
OF DACITE MASSES.
LLAO ROCK FLOW.
VITROPHYRIC DACITE.
This variety was collected from
the extreme southern edge of the lava flow, where it is necessarily very thin.
It is represented by Nos. 101 and 102. The hand specimens present the appearance
of a perfectly fresh vitrophyre in which the glass base, which constitutes at
least four-fifths of the entire mass, has a dark, greenish-gray color. In this
glass base are inclosed numerous phenocrysts of plagioclase that measure not
over 1 to 2 millimeters, and that are white and glassy and show hardly a trace
of cleavage. In addition to these are a very few minute, deep-brown to green and
black, resinous-lustered crystals that are hardly to be seen without a
magnifying glass, and that, in thin section, prove to be usually hypersthene or
hornblende. There are also to be noted a few angular fragments, compact, grayish
brown and dull lustered, measuring one-quarter inch to one inch or more in
diameter. These are referred to later under the head of nodular secretions. This
rock is more or less cracked in different directions owing to shrinkage in
cooling. The rock parting along these cracks, which sometimes gap, breaks into
very smooth flat surfaces that have a distinct gloss. Otherwise the fracture is
rather rough or small-concoidal, owing to the presence of the abundant
plagioclase phenocrysts. This vitrophyric dacite may also be streaked by more or
less parallel bands of grayish-looking pumice, as may be seen in a specimen
collected by the writer on the same spot. (No. 2013.1 of collection of H. B.
Patton.)
In thin section the glass
base appears to be clear and colorless and to be crowded with very sharp and
straight and remarkably even-sized microlites of augite that measure 0.003
millimeter wide by 0.02 to 0.04 millimeter long. (See fig. A of Pl.
XVIII, p. 132.) These microlites are too small to show any color, but the strong
refraction is evident, as well as double refraction and an extinction angle up
to 45° are also to be seen a very few opaque curved black trichites. The
straight, colorless augite microlites appear to be the same as are to be seen
sparingly developed in a thin section prepared by the United States Geological
Survey from the rhyolitic perlite of the Yellowstone National Park.a
They still more closely resemble in size, appearance, and numbers the straight
colorless microlites in the dacite from Lassen Peak, California, No. 82 of the
above-mentioned Educational Series.b
The close resemblance between the Llao Rock dacite and this dacite from Lassen
Peak will be again referred to in these pages.
aNo. 61 of the Educational
series of Rocks, described by J. P. Iddings in Bull. U. S. Geol. Survey
No. 150, p. 151.
bJ. S. Diller, op. cit., p. 217.
A few feldspar microlites occur
in No. 101, but almost none in No. 102. These are in part lath-shaped
plagioclase with very small extinction angles, but mainly short rectangular to
square, untwinned feldspar that usually extinguishes nearly parallel to the
sides. In No. 101 was seen a nearly rectangular section of feldspar that was not
larger than nor even as large as many of the feldspar microlites (0.05
millimeter long), in the center of which was a brown-glass inclusion with sides
parallel to the crystal edges and occupying at least one-third of the whole
crystal, itself inclosing a comparatively large air bubble. The extinction angle
measured to the longest side was 18° twinning was apparent. A reproduction of
this crystal may be seen in fig. G of Pl. XIV, p. 76.
Fluidal structure, which is
hardly to be seen in the hand specimen, is very conspicuous in the thin section,
owing to the more or less parallel arrangement of the microlites which occur in
flowing lines lapping around the phenocrysts of plagioclase, hypersthene and
hornblende, and of accessory magnetite and apatite. Hypersthene is scarce and
hornblende still more so. The latter occurs mainly in very small needles and
only occasionally in crystals comparable in size with the hypersthene. In one
instance a small crystal of hornblende with sharp crystal forms was observed
clearly inclosed in plagioclase. This is a decided exception to the rule that
hornblende in these dacites is the youngest of the phenocrysts with the possible
exception of augite.
The chemical analysis of No.
102 is given on page 140.
