• CENTRAL AMERICAN GEOLOGICAL WATER WAYS. 589 



of Mexico when its waters were last connected with the Pacific 

 Ocean, although the barrier has now been raised to seven hundred 

 and seventj-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 liummocks, 

 with a gravel plain in front (northern side). 



Glacial times, the excavation by the streams has produced only nar- 

 row canons, 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 

 difiiculties 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 othek Water Ways.- — The low country of the 

 Atrato-San Juan Yalley of Colombia has been mentioned as an im- 

 portant depression in the barrier between the Caribbean Sea and 



