The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta
by Howell Williams
Microscopic Petrography
Cinder Cones and Associated Flows
Forgotten Crater
The lavas and “cinders” described thus far are somewhat monotonous in their similarity, and it comes as a welcome relief to examine the flows erupted by Forgotten Crater. The cone itself is composed of normal olivine-bearing basaltic andesite scoria and lithic fragments. From its western base there escaped a long flow. Except near the source, this also is composed of olivine-bearing, vitrophyric and scoriaceous basaltic andesite (analysis no. 11). Approximately, its content is as follows: deep-brown glass, with dusty ore and trichites, 35 per cent; glass-charged, corroded phenocrysts of basic labradorite and microliths of andesine, 45 per cent; porphyritic and intergranular augite, 12 per cent; equal amounts of porphyritic olivine and hypersthene, 4 per cent; granular ore, 4 per cent. Within the flow lie ovoid xenoliths of Mazama andesite and more numerous inclusions of pre-Mazama lava.
