of Mexico when its waters were last connected with the Pacific Ocean, although the barrier has now been raised to seven hundred and seventy-six feet above the sea. Since the elevation of the land, which finally separated the Gulf of Mexico from the Pacific in post-
Fig. 8.—The Divide of the Isthmus of Tehuantepec at Chivela, eroded into hummocks, with a gravel plain in front (northern side).
Glacial times, the excavation by the streams has produced only narrow cañons, at most a mile or so in length and three hundred or four hundred feet in depth, while the gravel floors and terraces remain almost intact. Indeed, the final elevation seems to have been even later than the birth of Niagara Falls. Thus the separation of the Antillean and Pacific waters is found to have been much later than has hitherto been supposed. So recent is the geological canal of Tehuantepec, that it would be reasonable to suspect its existence even since the advent of man upon the earth, although the proof of his occupancy of the district and his utilization of the passage has not been found. Had the appearance of the barriers been retarded for only a little while longer (geologically speaking), the engineering difficulties of constructing interoceanic canals would have been avoided.
A sister canal to that described occurs a few miles toward the east, and probably similar narrow channels are found in the other half-dozen low passes of the region which have an altitude of about eight hundred feet.
The Atrato and other Water Ways.—The low country of the Atrato-San Juan Valley of Colombia has been mentioned as an important depression in the barrier between the Caribbean Sea and
