8 BULLETIN 134, UNITED STATES NATIONAL MUSEUM 



elevation of various transisthmian passes, irrespective of other data, 

 suggest the probable former isolation of the two greater Americas. 

 Some of the passes are less than 500 feet above sea level, and a 

 subsidence of 1,000 feet of the present continental mass would estab- 

 lish interocean connections at various points. Beginning on the 

 south some of these are marked by gaps in the mountains at the 

 source of the Rio Napipi. a tributary of the Rio Atrato, at the 

 Sucubti, an affluent of the Rio Chucunaque, at the Canal Zone, and 

 farther north at Lake Nicaragua and at the Isthmus of Tehuan- 

 tepec. Such a division would leave a chain of islands, several of the 

 more southern of which would be 3,000 to 4,000 feet high, and it 

 would isolate the high mountains of Costa Rica and Guatemala. 



"Geological investigations, especially those pursued in connection 

 with Panama Canal construction, indicate that oceanic waters did 

 in fact extend across, at least at the Canal Zone, during the Oligo- 

 cene period; but the date of land emergence has not been very 

 definitely determined. The slight depth of the water to submarine 

 escarpment far out along the coasts of Panama, and the present 

 rapid rate of erosion, indicate that the Isthmus was formerly much 

 broader than at present. The encroachment of the sea is well shown 

 along much of the northern coast line, w T here cliffs receive the full 

 battering effect of the waves swept in by the northerly trade winds. 

 Southerty winds are less dominant, but the southern coast is con- 

 stantly subjected to the erosive influence of tremendous tides." 



Rivers. — The more important rivers of the Caribbean slope of 

 southeastern Panama, as mentioned before, are the Mandinga and 

 the Cardi entering the Gulf of San Bias, and the Atrato flowing into 

 the Gulf of Darien. The mouths of the Atrato are obstructed by a 

 bar, but within the bar the channel is broad and clear to the con- 

 fluence of the Napipi and as far as Inibdo. The valley of the Atrato, 

 once an arm of the sea, has been gradually formed by the disintegra- 

 tion of the hills upon either side, and by the vast masses of vegetable 

 matter that yearly spring up and thrive. In the lower portion of 

 the valley this process is still going on, and there are vast swamps 

 extending for miles upon each side of the main channel, filled with 

 coarse granulate grass growing in many places so thickly as to pre- 

 vent the passage of boats and presenting the appearance of an 

 immense meadow ; yet underneath a deep strong current sets steadily 

 seaward. It is not before reaching some 60 miles from its mouth 

 that firm banks are found, but beyond that point they extend in 

 unvarying monotony 10 to 12 feet high, and without a sign of a 

 hill or highland in any part. On both sides of the river stretches a 

 level country covered with an unbroken forest filled with hard 

 woods, rubber trees, and valuable dye woods. Now and then may 

 be seen a dugout or flat-bottomed craft crowded with naked negroes, 



