Partial sections of the Paleozoic and Mesozoic sequence of southwestern New Mexico are exposed amid the Tertiary igneous rocks of the Tres Hermanas Mountains. Several hundred feet of the Silurian Fusselman Dolomite occur in a small fault block. Only the upper 360 feet of the Mississippian Escabrosa Limestone is exposed beneath 20-40 feet of the Paradise(?) Formation. The overlying Pennsylvanian is about 560 feet thick and consists of limestone, a few shale lenses, and a 60-foot-thick sandstone unit. The Wolfcampian Hueco Formation is at least 525 feet thick, is erosionally unconformable on the Pennsylvanian strata, and is composed of lower chert-limestone conglomerates and upper dark gray fossiliferous limestones. Lower Cretaceous rocks are more than 1,530 feet thick and onsist of four lithic units: (1) lower 375 feet of chert conglomerate, sandstone. pale red siltstone, and silty limestone; (2) 395 feet of massive limestone; (3) 425 feet of limestone conglomerate and reddish sandstone; and (4) upper 340 feet of sparsely fossiliferous limestone.