The region described in this paper lies along the Gulf coast of the Republic of Mexico in the States of Tamaulipas, Vera Cruz, and San Luis Potosi. The author visited this district during November and December, 1911, and was afforded every facility for travel by the officers of the Mexican Petroleum Company, Limited, of Delaware, on rail, steamer, and horse through several hundred miles of jungle, seeing and examining a large number of petroleum developments and seepages. For a portion of the trip the party was accompanied by Señor Ezequiel Ordonez, the distinguished Mexican geologist and Fellow of this Society, whose intimate knowledge of Mexican geology, and especially of the structural conditions in the petroleum fields of Mexico, proved of great value and assistance in the cursory study undertaken. The locations of the principal oil developments along the Gulf coast of northeastern Mexico, as well as . . .