20 Dutton Cliff Andesite Area

The Geology and Petrography of Crater Lake National Park, 1902

 PART I.

MOUNT MAZAMA.

LAVAS OF MOUNT MAZAMA.a

ANDESITES.

DUTTON CLIFF ANDESITE AREA.

In the broad divide between Sand and Sun creeks andesite plays a smaller part and shows but little variation. Near the middle of the divide, at an altitude of about 7,000 feet, there is an andesite (76) full of secretions (98). The andesite is quite normal, but the inclusions are basaltic. The same material with secretions occurs in the form of large bowlders near by, and a short distance to the west, in one of the head gulches of the West Fork of Sand Creek, it is overlain by a decidedly platy andesite (27). Similar andesite (50) full of secretions was seen near the edge of the water under Sun Creek Notch. The andesitic lavas of the south rim are much alike everywhere, with the exceptions noted above, and are represented by Nos. 50 and 46, the latter from Dutton Cliff, where it is capped by 10 feet of pumice.

Sand Creek, like its neighbor, is bordered by great cliffs, especially on the west. Ascending this creek one-half mile south of the lake rim, I collected a platy andesite (36) at the bottom. One hundred feet up No. 5 was taken, and 600 feet from the bottom of the canyon No. 23 was collected, while No. 37 came from the top of the hill. On this divide were collected the interesting ejected fragments (147-151) of dacitic material. About 2 miles from the notch the andesite has a marked platy structure, which is curved, showing broad surfaces and folds.

Farther southeast, at an altitude of about 6,200 feet, the cliffs of andesite (64) overlie dacite. Near the contact from which a great spring issues the andesite is black, as if from the presence of much amorphous matter, due to the sudden cooling of the under surface. Near by is a mass of very platy andesite closely associated with the underlying dacite. A short distance to the southeast, upon the same divide, the andesite (53) bears the same relation to underlying dacite (127). The approximate contact between the two rocks may be traced across to the canyon of Sun Creek.

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