168 BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



drainage parting of the Obispo, which flows into the Caribbean, and of 

 the Pio Grande, which flows into the Pacific, is not the site of a simple 

 axis, but is the remnant of what was once a circular central basin which 

 occupied the central region between the Culebra Pass and Mata Chin. 

 The Eiver Obispo was once a small tributary flowing into the southern 

 end of this. By capture through the headwater extension of a lateral 

 of the Chagres the lake was drained, and the Obispo converted into a 

 ramification of that stream. 



The Drainage System is Ancient. — Let us now consider the drainage 

 systems of the Isthmus which have been the chief factor in producing 

 this erosion. As shown by the accompanying map (Plate I.), this 

 drainage is exceedingly complex in its ramifications, the headwaters 

 of all the various streams interlocking through low cols between rugose 

 hill topography and the inherited courses of the stream ways are en- 

 tirely at variance with coastal directions. 



The Chagres may be well taken as a type of these streams. This 

 stream gathers into its trunk innumerable branches, which have a gen- 

 eral direction north and south in drainage valleys parallel to the coast. 

 While the character of the valleys in the upper portions of the streams 

 indicate long and continuous erosion, the antiquity of the stream is 

 further shown by the fact that it has been so long cut down to sea level 

 that the tides have drowned its lower portion for a distance of 11 miles 

 inland from the Caribbean. Its fall is only 4 feet per mile for 18 miles 

 southward in the direction of Panama, before it makes a sudden bend 

 eastward at Mata Chin towards Puerto Bello, and where, at a distance 

 of 30 miles from the Caribbean coast its bed is only 70 feet above the 

 level of the Atlantic Ocean. In general, distinct terraces or plains in- 

 dicative of serious alternations of level are not conspicuous, although 

 some of the playas above may be ancient benches. 



The Chagres above tide level is cutting into the ancient geologic sub- 

 structure, and shows no trace whatever of having followed any ancient 

 marine straits or passages, such as must have existed had the seas united 

 across this region in Pleistocene time, as has been fancied by some. 

 Whatever vicissitudes its lower portion may has suffered in its com- 

 bats with oceanic level, its headwater tributaries have been long and 

 continuously engaged in the task of degrading an ancient barrier, which 

 has persistently separated the two oceans since the beginning of the 

 present drainage. 



The other streams of the Isthmian region lying east of the Chagres 

 to the Andes all exhibit the same characters of drowned out mouths 



