Page:Popular Science Monthly Volume 53.djvu/601

Rh channel to one hundred and three feet above tide. Between Lake Nicaragua and the Pacific the divide is reduced to two hundred and thirty feet, but a subsidence of the land to three hundred feet or so would connect the lake basin with the Pacific Ocean by eight or ten straits between as many islands.

While most of the Isthmus of Panama is traversed by mountain ridges from one thousand to three thousand feet high, these are dissected so that five of the deeper depressions are reduced to an altitude of from five hundred and fifty-three to eleven hundred and forty-two feet, while the Chagres River Pass, along which the railway is built, has a natural altitude of two hundred and ninety-nine feet, although the artificial cut reduces it to two hundred and fifty-four feet.

Besides the Nicaraguan and Panaman depressions across Central America, the valley of the Atrato and San Juan (to Buenaventura) forms a third and equally interesting depression between the Caribbean and Pacific basins. This valley is about three hundred miles long (direct course) and crosses Colombia between the Coast Range and the great Cordillera. The Atrato Valley is from forty to sixty miles wide, and for long distances above its mouth (Gulf of Darien) it is forty feet deep. At two hundred miles from its mouth the river is only forty-seven feet above the sea. Above this point, the tributary Rio Quito is still navigable for steamers to a distance of two hundred and seventy-three miles from the Gulf of Darien, and for eight miles farther canoes freely ascend the stream Santa Monica. Between this point and the navigable waters of the San Juan, only three miles distant, upon the other side of the divide, the elevation is so low that during high water canoes can even pass from the branches of one river to those of the other. Here the divide is reduced to about a hundred and fifty feet above the sea. The whole valley of the Atrato suggests a comparatively recent connection between the Caribbean Sea and Pacific Ocean. Across the Coast Range, separating the Atrato Valley from the Pacific, there are several passes at an elevation of only one thousand feet or less.

The Atlantic and Pacific Coastal Plains.—Prom the seashore, the coastal plains, on the Atlantic side of the continental plateau, slowly rise until they abut against the edges of the table-lands. These plains may have a width of only a few miles, or they may extend for a distance of considerably more than a hundred miles from the Gulf of Mexico. Back of Vera Cruz the coastal plains have a breadth of more than fifty miles and reach an elevation of seventeen hundred feet, while in the Tehuantepec Isthmus they extend for a distance of eighty miles from the sea and deeply indent the plateau region (see map, Fig. 7, page 588). Similar coastal plains form the eastern border of Central America, extending sixty-five