Precambrian geology and ore deposits of the Pecos mine, San Miguel County, New Mexico
— William D. Riesmeyer and J. M. Robertson

Stimulated, in large part, by Giles' (1974) re-interpretation of the ore bodies at the Pecos mine as volcanogenic massive sulfide deposits, several workers have re-examined and remapped Precambrian rocks in much of the southern Sangre de Cristo Mountains. This mapping has delineated an extensive volcanic and sedimentary terrane, mainly in the headwater area of the Pecos River, that is known as the Pecos greenstone belt (Robertson and Moench, this guidebook). Riesmeyer (1978) discusses the Precambrian volcanic and sedimentary geology, and the ore deposits of the Pecos mine area in more detail than the present summary.