LAVAS OF MOUNT MAZAMA.a
CLOUD CAP DACITE FLOW.
Cloud Cap is on the eastern
crest of the rim and marks the point of departure of a stream of dacite which
spreads to the northeast. It forms a large part of Redcloud Cliff, which takes
its name from the reddish-yellow tuff or tuffaceous dacite that underlies the
principal flow. This flow, or, rather, group of flows—for it appears to be made
up of at least three streams—forms a prominent cliff for over half a mile along
the rim and has a thickness of over 300 feet. It appears to form one-third of
the inner slope of the crest. This flow presents a series of great cliffs about
its borders, especially on the northwest. The finest is upon the west side
nearly half a mile from the lake. It is 250 feet high and of great length,
smooth and polished as if by glaciers, but distinct striae could not be found.
The rock (118) is especially glassy, often banded with brown, and may be finely
spherulitic or lithophysal. The canyon heading between Cloud Cap and Scott Peak
presents cliffs which are less imposing. Upon the east side of this canyon at
the northern end of the flow the rock, although dacite, is more massive and
lacks the vitreous features of the main body of the flow near its source. To the
west this series of dacite flows, along the southern arm of Grotto Cove, may be
seen to overlie the adjacent streams of andesite. The base of the dacite flow
near the contact is glassy, with numerous spherulites and lithophysae.
Farther northeast, between the
forks of Bear Creek, is a small area of dacite, which was regarded as an
andesite in the field, but Dr. Patton finally relegated it to the dacites. The
regularity of the hill gave rise to the expectation that it would be found to be
a cinder cone, but this is not the case, as it is composed of solid dacitic
lava. Its sides are covered with lapilli of pumice, like that found elsewhere,
and upon the south bear a fine growth of yellow pine. There are a number of
other hills farther north which appear to be composed of the same material.