﻿600 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



2. Atlxntic and Gulf Coastal Plain. — -This plain extends beyond 

 Hio Grande to the Sierra Madre, Mexico, and as far southward as 

 Tampico. A narrow, more or less broken plain continues beyond 

 Vera Cruz to the lowland plain of Yucatan, where it meets the trans- 

 verse Oaxaca-Guerrero structural line. 



Throughout its extent, notwithstanding irregularities in surface 

 configuration, the Coastal Plain in general slopes from its landward 

 margin to the edge of the Continental Shelf. The inner margin 

 ranges from 300 to 600 feet in altitude between Maryland and central 

 Texas ; while in west Texas it attains a height of slightly more than 

 1,000 feet above sea level. 



3. Mexican Plateau. — At least four provinces of major rank are 

 recognized in the western Cordilleran region of the United States, 

 according to Ransome,* namely: (1) The Rocky Mountains, (2) the 

 Colorado Plateau, (3) the Nevada-Sonoran region, (4) the Pacific 

 ranges. Nos. 1 and 2 are parts of the Laramide mountain system; 

 No. 3 is the intermontane belt; and No. 4, the Pacific mountain 

 system. Fenneman dissents from this classification in that he refers 

 the Colorado Plateau to the Intermontane plateaus, along with the 

 Nevada-Sonoran region,^ and considers the Mexican "highland" as a 

 part of his Basin-and-Range province lying south of the Colorado 

 Plateau. Toward the south in trans-Pecos Texas the Colorado Pla- 

 teau and the Nevada-Sonoran region of Ransome are delimited by a 

 rather vague boundary from the Mexican Plateau, which Ransome 

 also considers a part of the Laramide mountain system. The Mexi- 

 can Plateau comprises the high plateaus and central mountains of 

 Mexico. Southward from Rio Grande, below the mouth of Pecos 

 River, it forms the western boundary of the Coastal Plain. The 

 boundary, according to Hayes (oral communication), is a fault scarp 

 which lies a little cast of Monterey and trends cast of south through 

 Ciudad Victoria to Misantla,' where volcanic mountains reach the 

 shore and interrupt the continuity of the plain. The province is ter- 

 minated on the south by a fault scarp beyond which are the east and 

 west trending structural axis of Michoacan, Guerrero, and Oaxaca. 



4. Oaxaca-Guerrero. — A structural axis extends through Michoacan, 

 Guerrero, and Oaxaca, almost at right angles to the trend of the 

 Mexican Plateau. The northern boundary of this province is the 

 escarpment at the, southern margin of the Mexican Plateau; the 

 western and southern boundary is the Pacific Ocean; while the 

 eastern boundary is the Isthmus of Tehuantapcc. It is thus set off 

 from the Mexican Plateau, and the Yucatan lowland. 



> Ransome, F. L., The Tertiarj' orogeny ol the North American Cordillera and its problems, Problem 

 of American geology, pp. 2S9-205, New Haven, 1915. 



' Fennamin, N. M., Physiographic divisions of the United States, Assoc. Amer. Geographers Bull, 

 vol. 6, p. 41, 1919. 



