Rocky Butte is a prominent, but isolated hill, with two volcanic vents. It is the core remnant of intrusive rock from a heavily eroded, late Pleistocene volcano that erupted basaltic andesite. Like many other vents in the Boring Lava Field, Rocky Butte consists of basaltic andesite with olivine phenocrysts, and it has a calc-alkalic composition. This basaltic andesite can be observed in exposures that extend from its summit park area to the roads below, as well as in a sill (a sheet of intrusive rock wedged between older layers of sedimentary rock, beds of volcanic lava, or tuff, or along the direction of layering in metamorphic rock) between layers of fluvial gravel from the Troutdale Formation on the Butte's eastern slope, within a flow scarp. Vug cavities were identified from rock in the historic quarry on the eastern side of Rocky Butte by scientists from the United States Geological Survey, including optically positive hornblende (also known as pargasite) with a pale brown color. They exhibit prismatic cleavage and extend for a few millimeters in length. Other minerals identified were dark green, pyroxene crystals with 70 percent hedenbergite and 30 percent diopside; white aggregates of tridymite; brown, granular olivine crystals with 80 percent forsterite and 20 percent fayalite; and tiny black, hexagonal plates of hematite with beveled edges. These minerals constitute a rare combination, and Trimble (1963) was unable to explain why free silica tridymite would be found together with low silica olivine.