The Sierra de Parras lies in southern Coahuila. The area mapped extends from a point about 4 miles east of Parras, eastward 45 miles to a point near the town of Fraile. The width of the range increases toward the east, varying from 8 miles, at the west end, to 12 miles in the region south of Huarichic and to 16 miles near the east end. The area mapped comprises about 550 square miles of mountainous country.
Mapping was done by pace and compass traverse. A relief map of the northern half of the range, between Parras and Huarichic, furnished by Señor Salvador Madero, was of considerable assistance. Sections were measured with a steel tape. Altitudes were determined by an aneroid barometer.
Preparation of this report was made possible by assistance received from the Geological Society of America . . .