DISTRIBUTION AND DESCRIPTION
OF DACITE MASSES.
CLOUD CAP FLOW.
This is an extensive flow that
starts at Cloud Cap and extends for about 3 miles in a northeasterly direction
with a breadth of about a mile. The rock specimens collected from this flow and
included in this description are Nos. 115, 116, 117, 118, 119, and 120. With the
exception of one distinctly lithoidal specimen (119), these are mainly very
glassy and also largely spherulitic dacites, closely resembling the Llao Rock
mass (as seen in Nos. 101, 102, and 104). The glassy portion of these rocks has
a light-gray color. Phenocrysts of white glassy plagioclase arc abundant, and
phenocrysts of the ferromagnesian minerals are scarce. Spherulites are to be
found in Nos. 116, 117, and 120, but they are conspicuously visible in the hand
specimen only of the last mentioned number. Here they occur from one-fourth to
one-half inch in diameter, and have a whitish to light-drab color. The larger
spherulites are more or less hollow and show on the cracked surface a fluted or
ribbed structure radiating from the center outward.
As this rock does not differ
very materially from the spherulitic dacites already described, a brief
description of the microscopic characters will suffice for present purposes. The
glassy portions in the spherulitic varieties, as well as in the entirely
vitrophyric groundmass of No. 118, contain characteristic streams of minute
straight and colorless augite microlites, like those in Nos. 101 and 102 of the
Llao Rock mass. Microlitic feldspars, in the form of lath-like plagioclase and
rectangular and apparently untwinned feldspar, occur in varying amounts, both in
the same and in different thin sections. Some streaks may be almost entirely
devoid of such feldspar microlites, while others may be crowded thickly with
them, while between these two extremes there appears every conceivable
intermediate stage. In two or three thin sections but more noticeably in No.
117, the untwinned feldspar, or more properly the microlitic feldspar, that does
not appear to have the albitic twinning, occurs in very thin leaves that show
striking Carlsbad twinning. The two individuals that make up one of these twins
have each of them simple quadrilateral form, and the leaves lie over each other
joined by the clinopinacoid, which is also the plane parallel to which the
little leaves are extended. Professor Rosenbusch refers to such twinned
microlitic feldspars as occurring in rhyolite pitchstone and rhyolite obsidian.
No. 116, which shows in the hand specimen very strongly developed fluidal
structure, consists of alternating, parallel streaks of vitrophyric, spherulitic
dacite, and of a more lithoidal dacite free from spherulitic inclusions. The
lithoidal streaks have the plagioclase laths very strongly developed and at
times bear a marked resemblance to some of the Crater Lake andesitic rocks. On
the other hand, No. 115 has a brown glass groundmass, inclosing numberless
somewhat granulated augite microlites, and is almost free from feldspar
microlites of any description. Slender, straight, and curved opaque black
trichites, as well as a very little black ore in minutest grains, may also be
mentioned as occurring in some of these specimens.
The spherulites differ somewhat
from those in No. 104, in that zones of growth are not so plainly developed.
There is, however, often an inner, coarse-grained portion occupying the greater
part of the spherulite and an outer, denser, and usually deeper stained zone.
The inner coarser part is made up of radiating shreds that show both positive
and negative extinctions; usually, however, positive. They also have a tendency
to fork at small angles and do not extinguish always parallel.
The phenocrysts do not present
any characteristics peculiarly different from those common to the other dacites
with the possible exception of hornblende, which is rather more abundant than
elsewhere. It has mostly a greenish-brown color, but also occurs reddish-brown
in No. 116. It is absent only in No. 115. It is of interest to note in this
connection that, although these rocks contain more hornblende than do the other
dacites of Crater Lake, still the actual amount is very small indeed. As far as
may be judged, a thin section from this flow does not contain one-fiftieth or
perhaps one-hundredth part as much hornblende as does the dacite from Lassen
Peak, to which reference is made above and with which these Crater Lake
rhyolites are closely allied, it should further be stated that occasional nests
of older secretions are to be seen consisting of plagioclase, hypersthene,
hornblende, augite, and magnetite, in which hornblende is apt to be very
abundant. Zircon, which is so often reported as occurring in such rocks, appears
to be a rare accessory mineral in the Crater Lake rocks of all types. It was
noticed in a single crystal in No. 117.