CENTRAL AMERICAN GEOLOGICAL WATER WAYS. 581 



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 dis- 

 sected 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 Carib- 

 bean 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 re- 

 duced 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 ex- 

 tend 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 



