The felsic shield-building stage began with the eruption of phonolite, trachyte, phonolitic trachyte, quartz-trachyte and rhyolite lava. Subsequent volcanism of the pre-explosive phase produced a basal sequence of aphyric trachyte lava flows and domes with minor altered flow-banded rhyolite, sulfide-bearing rhyolitic tuffs and a few thin hawaiite lava flows. This activity was concentrated at the summit of the volcano as shown by the increasing thickness of volcanic material towards the summit. A hydrothermally altered aphyric trachyte dike, which forms a narrow ridge linking Mount Downton and Itcha Mountain, might have been the source for these eruptions.