This report presents the writer’s interpretation of the geology of a complex mountain area contiguous to a well-populated and economically important agricultural region (Figs. 1 and 2). The Laguna District, the most important cotton-raising region of Mexico, owes its fertility to the Nazas River, which, entering from the southwest, annually brings a new supply of rich alluvium to the fields. Many haciendas and several large towns located in the district draw their livelihood from the soil and from the mines of the neighboring mountain ranges. The rocks that underlie the alluvial plain are exposed in the mountains bordering it on the west. Especially along the valley of the Nazas, which crosses the strike of the mountain ranges, the sequence of rock formations is well exhibited. Because of the large number of people who live within sight of these mountains and because of the importance of the . . .