The mineral assemblage in the Ellensburg formation is unusual in many respects. The heavies are restricted exclusively to hornblende and magnetite. The lights are essentially glass and plagioclase. All of the minerals are remarkably euhedral although evidence points to sources up to 50 miles or more from their present position. The uniform mineral content persists over an area of many hundreds, if not thousands, of square miles and may prove to be an aid in working out the stratigraphy of central Washington.