Page:Popular Science Monthly Volume 53.djvu/612

592 physical testimony of the recent great changes which have occurred in the Central American and West Indian regions.

—The Reconstruction of the Antillean Continent, and The West Indian Bridge between North and South America, have been largely based upon the testimony of the great river channels being traceable as drowned valleys of natural proportions across the submerged plateaus or margins of the land to the floors of the Gulf and Caribbean basins, which are characterized by large plains. This fundamental feature is supported by a great array of facts collected by the writer and described in the papers named. It was during the early part of the Pleistocene period that the West Indies were united into a high plateau and the floors of the sea basins were transformed into plains, to the margins of which the river valleys have been traced. The continuance of the river valleys to the floors of the sea basins is the result of their formation upon the surface of the land before the region was submerged beneath the oceanic waters. Satisfactory as was the evidence of the high continental elevation to the east, the Central American region now appears as a barrier against its former drainage to the Pacific, thus producing inclosed sea basins. On a smaller scale, there is a perfect analogy in the basin of Lake Ontario, which is only the valley of the St. Lawrence, mostly closed by warping of the earth's crust so as to raise up a barrier of several hundred feet in place of thousands of feet, as is the case in Central America. It was very significant that the depressions in the Mexican and Central American table-lands correspond only to the extensions of the West Indian basins, thus suggesting the location of their outlet. But the discovery of the recent general elevation of the American barrier to thousands of feet, and the preservation of the last water ways across the divide, are found to show that the barrier did not obtain at the time when the Antillean basins, according to the hypothesis, should have been drained into the Pacific. Thus the low altitude of Central America at the time of the high continental elevation to the east and the presence of the recent water ways to the Pacific Ocean, lately discovered, complete, in a manner, the story of the great oscillations of land and sea in the regions between North and South America.

Of the two miles or two miles and a half in height of the Central American barrier above the floor of the sea basins, probably half that amount has been produced by gentle warping of the earth's crust, amounting to only a few feet per mile, like that across the outlet