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 constituto 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 coasta. 
The drainage of the Isthmian region is somewhat complicated and 
difficult to describe, and is about equally distributed between the two 
oceans," 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 
ealled “drowned rivers.” Technically, this drainage may be defined as 
ancient, mature, and autogenous, consisting of deeply incised head water 
ramifications, “drowned " in their lower courses towards the sea. 
In the eastern. (South American) half of the Tsthmus 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 
direotions into wide trunks leading to the sea. Тһе Atrato, rising in 
the Republie 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. D. 2 
