The Tascotal Mesa quadrangle is in southeastern Presidio and southwestern Brewster counties, Trans Pecos Texas. Bandera Mesa and Tascotal Mesa are reached most easily from Marfa, 50 miles north, by the Marfa-Lajitas road which traverses the quadrangle from north to south. Green Valley in the northeastern part of the area is most easily reached from Marfa by the 02 Ranch road in Paradise Valley; an alternate route is from Alpine by the Terlingua highway and the 02 Ranch road. This investigation is primarily concerned with a thick succession of volcanic tuff and and related sediments with intercalated lava flows which Goldich and Elms (1949, p. 1143) named the Buck Hill volcanic series. Six months was spent in the field during the summer of 1949 and the spring of 1950. Mapping was done on aerial photographs and later transferred to the U.S. Geological Survey topographic quadrangle map (ed., 1944; surveyed in co-operation with the War Department).