HILL : GEOLOGY OF THE ISTHMUS OF PANAMA. 165 



The continuity of the rugged coast line is broken at frequent inter- 

 vals by swamp lands usually adjacent to the outlet of some river. 

 These swamp levels extend far inland and constitute a large area of 

 the entire Isthmian topography. 



The country adjacent to the Atrato far southward towards the 

 Equator consists of low swampy ground of this character. This is 

 also true of extensive stretches up the principal tributaries of the 

 Tuyra, the Bayano, and the Chagres. Similar patches of low swampy 

 land are also found at intervals along both coasts. 



The drainage of the Isthmian region is somewhat complicated and 

 difficult to describe, and is about equally distributed between the two 

 oceans, 1 but does not part from a continuous structurally defined water 

 shed. This drainage consists of streams of great age, the headwaters 

 of which are so minutely ramified that, in the course of their past ex- 

 istence, they have long since etched over every portion of the original 

 surface. These numerous branching ramifications gather into a few 

 arterial channels, emptying into the sea, usually reaching its level 

 so far inland that they become tidal rivers, oftentimes for a distance 

 nearly half way across the Isthmus, thus constituting what would be 

 called " drowned rivers." Technically, this drainage may be defined as 

 ancient, mature, and autogenous, consisting of deeply incised headwater 

 ramifications, "drowned " in their lower courses towards the sea. 



In the eastern (South American) half of the Isthmus the drainage 

 consists of a few principal arterial streams, whose interlocking head- 

 water ramifications radiate out over great circular areas like innu- 

 merable branches of a low wide spreading tree, gathering from all 

 directions into wide trunks leading to the sea. The Atrato, rising in 

 the Republic of Colombia about at the second meridian, flows almost 

 due north into the Gulf of Darien for a distance of nearly 600 miles, and 

 has a fall of less than one foot per mile for its entire distance. West of 

 this river the Tuyra drains most of the country as far as Panama Bay, 

 carrying the waters into the Pacific. This stream and its tributaries 

 form a remarkably complex system, the digitated headwater ramifica- 

 tions radiating in all directions, reaching almost to the basin of the 

 Atrato on the east and the Atlantic on the north. This is by far the 



1 The details of the drainage, excepting that of the principal arterial trunks, 

 are but poorly known. This is well exemplified on Wiess's map, where all the 

 headwater ramifications are sketched in a conventional manner except in the area 

 adjacent to the Panama Railway surveys, where the characteristic drainage is well 

 shown. 



vol xxviii. — no. 5. 2 



