Close to its source this western stream of lava and the short flow which spilled over the south rim of the crater show a peculiar banded, composite character. Layers of dark olivine-bearing basaltic andesite alternate with layers of pale-gray, highly pumiceous lava from a few millimeters to a few inches in width. Xenoliths of cristobalite-rich, pre-Mazama andesite are common to both. In the field the dark and light layers are sharply defined, and in thin section the contrast is no less striking. Whereas the glass in the dark lava is deep brown, the glass in the pale bands is colorless and extremely vesicular, is commonly traversed by perlitic cracks, and carries swarms of belonites, probably of oligoclase. In this glass, which constitutes approximately two-thirds of the pale lava, lie zoned phenocrysts of andesine-labradorite (up to 20 per cent), and crystals of augite (3-9 per cent), hypersthene (2-3 per cent), ore (2 per cent), and brown oxyhornblende (I per cent). Seen apart from the associated olivine-bearing, dark andesite, this pale lava would be classed immediately with the dacites erupted from the Northern Arc of Vents.
It seems, therefore, that during the closing stages of its activity, Forgotten Crater, which had formerly erupted nothing but olivine-bearing basaltic andesite, began to erupt, in addition and simultaneously, dacitic lava distinguished by colorless glass, more acid feldspar, the absence of olivine, and the presence of hornblende. These two magmas showed no tendency to mix though they were finely interlaminated.
This eruption of two distinct types of magma from a common source serves again to emphasize the fact that throughout most of its long history Mount Mazama, like Mount Shasta, erupted little but hypersthene andesite. During its waning stages, on the other hand, the principal products were dacite and olivine-bearing basaltic andesite and basalt.
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