Sedimentary and. volcanic r•cks of probable Tertiary age overlie Cretaceeus sedimentary reeks in the Cerro Coronel Area, Northwestern Baja Califemia, Mexic•. Th• Tertiary(?) sequence is virtually flat-lying and cansists af approxiruttely 200 feet of sandst•n• and cenglemerate everlain by 600 to 700 feet ef basaltic flows and. interbedded velcanic breccia. Assignment efTertiary age te the strata in question is based on lith•legy and stratigraphic positien. The conglomerate of tho sequence centains 50 to 60 per cent rhyelitic, dacitic, and dark homfelsicclasts, These clasts are characteristic of Tertiary and yeunger conglemerates ef the Pacific ceast of nerthern Baja California and seuthern Califernia. A mere precise age issuggested by oorrelatien with similar volcanic units in La Mision area are ever-lain by marine fossilifeNus sandstenes and mudstenes •f Miocene age. On this basis, the Cerr• Cerenel sedimentary and v•lcanic sequence is no younger than mid-Tertiary; similarly, the unit is ne elder than Eocene on the basis of lithelogic correlation with Eecene c•nglemerates of San Diego Ceunty. Several uncenfermities are apparent in th• area. An uncenformity exhibiting 75 to 200 feet ef relief is developed en a rock unit thought te b• cerrelative with the lower Cretaceous AlisitesFon.nation. A discenfennity eccurs between upper Cretaceous sedimentary reeks and. overlying Tertiary(?) racks. A third uncenfermity occurs between th• sedimentary andvolcanic units ef the Tertiary sequence; it exhibits a maximum ef 100 feet 11f relief.