PART I.

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MOUNT MAZAMA.

LAVAS OF MOUNT MAZAMA.^a

ANDESITES.

ROUND TOP ANDESITE AREA.

The Round Top area is simply a part left uncovered by two dacite flows, one on each side, and the glacial material to the northeastward. It embraces upon the surface of the rim two flows, forming prominent cliffs, the easternmost of which is shown in Pl. VII, B. The western palisade flow fills an old valley to the water's edge, and, like its neighbor, has a thickness of over 300 feet. The lower 10 feet of this flow, especially that of the eastern palisade, where it comes in contact (17) with its bed, is glassy. It is much broken below, and lies in places upon conglomerate. Parts of it are very platy, excepting in the glassy portion below, and an imperfect jointed structure extends through the mass. It thins out rapidly on both sides to an edge, and is overlain by a thin layer of tuffaceous dacite, a later flow of which appears at the left in Pl. VII B.