Igneous rocks of the Sierra Madre Occidental have been studied along two traverses across the range. One is at lat 24°N between Mazatlán and Durango City, where contiguous mapped areas extend across the Sierra; the other is near lat 28°N, where several separate areas west and north of Chihuahua City have been mapped. In these regions the Sierra contains two vast and largely coextensive igneous sequences, both calc-alkalic and both including ignimbrites. The older sequence of rocks, which ranges in age from 45 m.y. to at least 100 m.y., is characterized by abundant batholithic as well as volcanic rocks and...