Page:Popular Science Monthly Volume 53.djvu/36

 doors of the Gulf of Mexico and the Caribbean Sea, to which depth the drowned valleys can be traced, were continental plains, like those of the Mississippi or Amazon of the present day, with perhaps some shallow lakes or small seas. The backbone of this West Indian bridge was near the Atlantic side, and remains of it are seen in the ridge dissected during the epoch of high elevation by the rapidly descending streams, which now constitute the chain of the Windward Islands.

The plains now forming the floors of the Gulf of Mexico and the Honduras and Caribbean Seas were apparently drained into the Pacific Ocean across the Tehuantepec Isthmus, Honduras, Nicaragua, Panama (see map, page 000), and other low depressions farther south. The writer′s recent explorations in the Tehuantepec Isthmus confirm this hypothesis. Plateaus of Mexico and Central America, rising to from six thousand to ten thousand feet, are there reduced in height for a distance of more than sixty miles, so that we find half a dozen passes as low as eight hundred feet above the sea. During the earlier period of the elevation of the Antillean region the Tehuantepec Isthmus was a strait in which a deep-water fauna was living. Later the strait was transformed into land troughs, fragments of the old base-level floor of which are still extant, and through them narrow geological canals were formed when the low Mexican plains had again sunk beneath the gulf waters. This question of the elevation of the Mexican and Central American barriers would carry us beyond the limits of the present paper, so that all that can now be said of them is that they were elevated at a very recent date, corresponding to the subsidence of the West Indian region, to heights reaching from six thousand to even more than ten thousand feet. This elevation was a sort of compensation in the terrestrial balance.

.—Over the West Indian region there are many widely distributed geological formations which were acumulated in early Miocene times. During the following late Miocene and Pliocene periods these formations were lifted above the sea and became lands of great extent. This elevated condition continued so long that the country became enormously denuded, and was reduced to valleys and low base-level plains, some of which now appear to constitute the gradation plateaus beneath the sea. On the sinking of the land, after the Mio-Pliocene period of elevation, new deposits (the Lafayette) were accumulated in the valleys, the age of which is provisionally placed at the end of the Pliocene, although some might regard it as early Pleistocene, for there is no sharp line of demarcation characterizing the limits of these formations. But these deposits immediately underlie those of the Glacial period of the north. Subsequently the land of the West