The Eocene contacts as they appear along the Rio Grande in Texas have been traced southward into Mexico as far as the Rio San Juan. The nomenclature and contacts used are with slight modifications the same as those at present accepted by the United States Geological Survey. A stratigraphic reconnaissance has been made which correlates some of the results of several rather extensive and detailed mapping operations carried on by the writers during the past 7 years.

Cross sections at six places along the belt covered by the outcrop of these beds reveal a progressive thickening toward the south of the Midway and Wilcox sections exposed. The Carrizo sandstone is shown to be thickening rapidly toward the south as far as the Monterrey-Laredo railroad from which point a marked thinning is noted toward the south. The Mount Selman becomes much thicker toward the south and is characterized by more sandstone beds than it has in the border province of Texas. The Yegua and Fayette formations also have more sandstones here than in Webb and Zapata counties, Texas, but these begin to thin toward the south and many of them disappear before the Rio San Juan is reached.

For convenience in locating the contacts and making a study of the excellent sections exposed, distances in miles and in kilometers are given in connection with some of the stratigraphic sections traversed by roads or railroads. Paleontologic studies of surface samples and well cuttings made on the basis of Texas type sections seem generally to corroborate the position of the contacts carried down by geologic mapping.