Some thin flows and the basal parts of a few thick flows are distinguished from the dominant pilotaxitic lavas by the presence of abundant brown glass. To this they owe their darker color. Their texture varies from hyalopilitic to vitrophyric.
Mention should be made in conclusion of some exceptional types of andesite. On the lower, western slopes of the volcano, the normal hypersthene andesites give way to dark and generally vesicular flows either exactly or almost identical with the pre-Mazama lavas already described. In brief, these are hyalopilitic basaltic andesites characterized by a greater proportion of porphyritic olivine than the normal lavas of Mazama, and by the presence of considerable cristobalite. Similar basaltic andesites may be seen at the south end of Sun Meadows. Some of these olivine-bearing lavas are interbedded with hypersthene andesite, but most are products of late eruptions. In view of their similarity to the pre-Mazama lavas, little reliance should be placed on the contacts indicated on the map, plate 3.
Equal difficulty was experienced in the attempt to determine whether some of the paler, more vesicular and glassy, well banded lavas should be classed as andesite or dacite. In default of chemical analyses, the decisions based on field appearance and on microscopic examination can only be tentative. The problematical lavas are as follows: among those grouped with the dacites are the flows on the flanks of Cloudcap and those of Scott Bluffs; among those mapped as andesites are (1) the highly vesicular, tridymite-rich lavas on the crest of Munson Ridge, above Government Headquarters, (2) the crumbly, white, almost pumiceous, tridymite-rich lavas which in many places border the Rim Road where it sweeps across Vidae Ridge, and (3) the finely laminated, cristobalite-bearing, glassy lavas on the west side of Sand Creek valley, near the Lost Creek Ranger Station. Other “dacitic andesites” are listed by Patton.
The dacitic andesites just mentioned, and the basaltic andesites referred to in the preceding paragraph, though widely distributed, constitute only a very small fraction of the total products of Mount Mazama. It should be borne in mind, moreover, that almost all of them were erupted during the later history of the volcano, immediately prior to the eruptions of dacite from the Northern Arc of Vents and the formation of the parasitic cinder cones. The earlier and main activity of Mount Mazama was marked by eruption of little but normal hypersthene andesite.
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