The study area is situated along the southern edge of the Superstition Mountains approximately 40 miles east of the greater Phoenix metropolitan area (Figure 1). Geology is dominated by mid-Tertiary volcanic rocks of the Superior volcanic field (Ransome, 1903), and these rocks depostionally overlie a crystalline basement of early Proterozoic Pinal Schist intruded by middle Proterozoic granitoids. In some areas a relatively thin sequence of the Middle Proterozoic Apache Group occurs along the contact between these two rock types.