Page:Popular Science Monthly Volume 53.djvu/598

578 epochs since the early Tertiary period, and the Last was so recent as to place it almost in modern times.

In searching for the geological water ways, the phenomena of the recent elevation of Mexico and Central America became so apparent as to establish the theory that the uplifting of the American barrier to its great altitude occurred principally since the period when the West Indian islands formed a high plateau bridge between the eastern portions of the two American continents (as described by the writer in The West Indian Bridge between North and South America). Indeed, the discovery of the recent elevation of Central America and Mexico forms a most important sequel to the story of the now submerged West Indian lands, for it shows that Central America was low at the time when the Antilles stood at a great height—though it has subsequently been elevated, while the eastern region has been largely submerged. The discoveries of these great changes of level in recent periods are the necessary complementary phenomena of those found in the Antillean region.

—The Table-Lands.—Much the larger area of Mexico and Central America is occupied by plateaus from six to eight thousand feet above the sea. Indeed, some of them have an altitude of over ten thousand feet. These elevated table-lands rise abruptly for thousands of feet above the inner margins of the coastal plains, which gradually slope upward from the seashores. The elevated plains, traversed by many mountain ridges, are, to the eye, apparently level. Out of their floors there also rise numerous island-like hills called cerros, as well as great volcanic cones. Thus the volcanoes of Popocatepetl and Iztaccihuatl, capped with perpetual snow, tower to a height of eight thousand feet or more above the Mexican plateau, as shown in Fig. 1.

The floors of the plateaus are substantially base levels of erosion—that is to say, the more ancient land surfaces were in olden days ground down by the action of the rains and rills to such low elevations above the sea that the streams could not further deepen their channels, whereupon the mountains were worn down into rolling plains. The hills and the mountains traversing the table-lands are often only the remnants of higher plains which have, so far, escaped destruction. Thus these elevated plateaus themselves demonstrate their subsequent great elevation above the sea.

In proceeding from Mexico into Central America, the high plateaus are not only necessarily of smaller extent, but they are more broken, and on the Isthmus of Panama they are almost replaced by mountain ridges rising from one to three thousand feet, except along a few lower passes.