Where cristobalite is rare or absent, its place is taken by tridymite. Beautiful examples of tridymite-rich inclusions, in which the fans reach 0.5 mm. in length, occur in the topmost andesite on Sentinel Point and in the thick flow near the caldera rim east of the Wineglass. Whether tridymite or cristobalite is present, the silica was already concentrated in the inclusions before they were incorporated in the surrounding lava. Presumably the inclusions represent partly or wholly crystallized fragments torn from the walls of the conduit by rising lava. This opinion accords with the observation that conduit fillings and dikes on other Cascade volcanoes are characterized by unusual richness in the silica minerals. Hornblende-rich inclusions, like those in the dacites of the Lassen region, have not been seen among the andesites of Mount Mazama, and even among the dacites they are far subordinate to the pyroxenic varieties just described. In addition to the cognate nodules, there are occasional xenoliths of older lavas, but these, being unaltered, show no peculiarities worthy of special description.
***previous*** — ***next***
