Page:Popular Science Monthly Volume 53.djvu/599

Rh Interruptions of the Plateaus corresponding to the Antillean Basins.—In the paper upon The West Indian Bridge between North and South America it was pointed out that the Antillean seas formed three distinct basins—the Gulf of Mexico, the Sea of Honduras (between Cuba, Haiti, and Jamaica), and the Caribbean Sea, although the two latter are united at their surface under the name of the Caribbean Sea. In the investigations of the river valleys, now drowned beneath the West Indian waters, it was found that they were traceable to the floors of the three basins just mentioned. Accordingly, the hypothesis that these three basins were formerly drained into the Pacific Ocean called for corresponding depressions

 showing the plains out of which rise isolated cerros, with the volcanic cones of Popocatepetl and Iztaccihuatl in the background.

across Central America and Mexico; and only such are found to occur (as shown in map. Fig. 2), although the continental barriers are now raised to considerable heights above the level of the sea. Thus the Mexican plateau rapidly narrows to a breadth of only twenty-five miles in the Tehuantepec Isthmus, where for a distance of sixty or eighty miles it is reduced so that the higher points do not exceed four thousand feet, and for a distance of perhaps over twenty-five miles the ridges do not reach an altitude of more than two thousand feet, with a repetition of base levels of erosion only nine hundred or a thousand feet in altitude among them. Indeed, these lower divides are further reduced at seven or eight passes which are only about eight hundred feet above the sea. Beyond these low depressions there are higher passes in the isthmus having an altitude of about twenty-seven hundred feet with accompanying plains. The Tehuantepec depression through the Mexican plateau