The Geology and Petrography of Crater Lake National Park, 1902
LAVAS OF MOUNT MAZAMA.a
UNION PEAK ANDESITE AREA.
Toward the southern border of the area mapped, near Pole Bridge Creek, is a mass of andesite, forming a bold, rocky point, which has been strongly glaciated on the side toward Crater Lake. The two types of andesite (41 and 100) found here do not look entirely alike, yet the rocks appear to be the same. The first is andesitic in appearance and the second somewhat dacitic. This is part of the large area which is outlined as including Union Peak, but is recognized as containing much basalt. Although the summit of Union Peak is hypersthene-andesite, the middle and lower slopes upon the eastern side are composed of basalt occasionally rich in olivine. There was no opportunity to map the southern and western borders of the park in as great detail as the other portions.