Page:Popular Science Monthly Volume 53.djvu/31

Rh depths from twenty-five hundred to thirty-five hundred feet. From its margin the edge of the continent drops abruptly to depths of twelve thousand feet or more. On the western side of Florida the drowned plains gradually slope down to about three hundred feet, beyond which there is an abrupt descent to the abyss of the Gulf of Mexico. The Yucatan plains extend as broad submerged banks to about three hundred feet beneath the sea, and the sea floor then falls rapidly to that of the Gulf of Mexico, at twelve thousand feet. Such slightly submerged plains of great breadth occur on the banks between Honduras and Jamaica. The Windward Islands are fragments of a plateau which an emergence of less than three thousand feet would unite into one body of land. Both to the east and west of Jamaica there are plateaus depressed to between three thousand and four thousand feet. Still lower plateaus are indicated in the Caribbean Sea, which reaches to a depth of fifteen thousand feet.

From the existence of these submerged plateaus it becomes apparent that a change of elevation of from two thousand to four thousand feet would not merely unite Cuba and Florida, but would greatly enlarge the West Indian lands, and almost connect North and South America. Such a change of elevation would nearly barricade the Antillean water into three basins—the Caribbean Sea, the Sea of Honduras, and the Gulf of Mexico. The Sea of Honduras, between Cuba and Jamaica, reaches to the phenomenal depth of twenty thousand feet, in the form of a long, narrow channel. The structure of these sea basins, at various depths, has been mapped by many writers, but the character of the channels which dissect the drowned plateaus had been almost entirely passed over until the appearance of the writer's previous papers.

—The greater number of the submerged valleys discovered are found to be continuations of buried channels of modern rivers. In some cases the valleys have been so completely submerged that even the divides between those trending in opposite directions have also been drowned—as, for example, the straits of Florida. These drowned valleys have been traced across the submarine plateaus and terminate in enlarged embayments indenting the margin of the continental mass. A few great submarine valleys are found parallel with the mountain ranges. The continuation of many land valleys beneath the sea, and others completely submerged, is shown in the accompanying map (Plate III), and attention may now be called to a few notable examples.