The Harrington Peak Quadrangle is located within the Caribou National Forest of southeast Idaho. Within this quadrangle are outcrops of olivine trachyte of Pliocene(?) age overlying sedimentary rocks of Mississippian to Tertiary age. The region contains thrust faults and later normal faults (generally trending north-south} formed during Basin and Range extension. The Largest outcrop of olivine trachyte (approximately1 1/2 X 3 km) probably formed as the result of a fissure eruption. Two other outcrop areas show evidence of being sites of local extrusion. Whole-rock chemical analyses revealed the olivine trachyte to have moderate amounts of SiO2 and Al2O3, high MgO and CaO, and K2O in excess over Na2O (approximately 2:1). Mineralogical characteristics include microphenocrysts of Mg-rich olivine and diopsidic augite in a groundmass of Ba-rich sanidine, diopsidic augite, Fe-Ti oxides, and less commonly phlogopite and/or plagioclase. The olivine trachyte closely resembles the ciminites from the Viterbo region of Italy and has some petrological and mineralogical similarities to many other continental potassic volcanic rocks. The olivine trachyte may have formed by partial melting of a heterogenous mica peridotite mantle source enriched in incompatible elements during a previous tectonic event.