PART I.

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MOUNT MAZAMA.

LAVAS OF MOUNT MAZAMA.^a

DACITES.

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.