History of the oil and gas industry -- Topography -- Surface stratigraphy -- Subsurface stratigraphy -- Geologic structure -- Origin of the oil and gas sands -- Oil and gas fields and pools by sands -- Secondary recovery methods -- Future shallow exploration -- Deep exploration. and Plate 3. Map delineating producing areas of oil and gas sands Sheffield quadrangle, Pa. (Southern half) -- Plate 3. Northern half -- Plate 4. Section A A' -- Plate 5. Section D D' -- Plate 8. Isopach map of Clarendon and Balltown sands Sheffield quadrangle, Pa. -- Plate 9. Isopach map of Cherry Grove and Cooper sands Sheffield quadrangle, Pa.