The area with which this paper is concerned is in the southwestern part of the state of Coahuila, Mexico (Fig. 1). It lies in the Mexican Plateau Division of the Basin and Range Province, and is characterized by dissected broad upland areas separated by plains of aggradation. The semi-arid climate has discouraged settlement and scientific investigation, and only a small part has been more than casually visited, although the area presents several highly interesting geological problems. Within its borders are the vestiges of the Jurassic-Neocomian landmass Llanoria, the position of which controlled sedimentation throughout the Cretaceous, and affected the structural trends of the mountains formed at the close of that period. The Paleozoic stratigraphy of the area records marine sedimentation, and a possible glacial period, followed by deformation and intrusion. It throws additional light on the position of the ancient landmass and its bordering geosyncline.
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