Page:Popular Science Monthly Volume 53.djvu/35

Rh crossing the submerged coastal plains is no greater than that illustrated by those of the land.

The resemblance between the terrestrial and submarine valleys is even more striking in the case of their tributaries, which come from various directions. Where there are short tributaries descending from submerged plateaus, they have also the form of amphitheaters like those dissecting the margins of table-lands.

The slopes of the submarine channels consist of a series of steps, like the gradation plains descending from high table-lands. These steps represent long pauses in the elevation of the plateaus. For comparison of the declivity of the drowned valleys with those of the land, sections are given on pages 21 and 22 in Figs. 5 to 9, all drawn to the same scale. The declivities of some of these gradation plains do not exceed a foot per mile—that is, these slopes are as low as those of continental valleys which are reduced to the base level of erosion. While much is yet to be learned of the detailed features between the submarine steps, we already know that their mean declivity is less abrupt than that of land valleys descending from the Mexican plateaus. The submerged steps appear to have the same origin as those on the border of table-lands—that is to say, they were formed during the pauses in the terrestrial oscillations when the now drowned continental plateaus formed table-lands.

The deep channels crossing the submerged plateaus for a distance of two hundred or three hundred miles, with a depth of from two thousand to six thousand feet—and among the Bahama banks to even greater proportions—show a close resemblance to the Colorado Valley and Cañon.

—From the apparently complete analogy between the characteristics of the land and submarine valleys or channels—namely, (1) the submarine valleys being continuations of those of the continent or islands; (2) both having tributaries converging from every possible direction; (3) both classes of valleys having their magnitude of corresponding proportions, with (4) similar great canons and amphitheaterlike tributaries; (5) both terrestrial and submarine channels with similar gradation plains and steps characterizing their slopes; (6) without obstructing barriers—the conclusion is reached that the depths of the submarine channels may be used as yardsticks to show that the land lately stood nearly as high as the valleys are deep.

Applied to the West Indian region and adjacent parts of the continent, it would thus appear that these regions stood from ten thousand to twelve thousand feet, or in some localities fourteen thousand feet, higher than now. The West Indian bridge reached a height of from two to more than two and a half miles, while the