HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 169 
and actively working headwaters. А careful perusal of the explorations 
of these rivers shows exactly the same conditions to exist along their 
courses. They are all ancient watercourses up which the tide extends 
far inland, while the headwaters are cutting into the mountainous bar- 
riers. The Atrato, Tuyra, Bayano, and other streams explored, do not 
flow in straits formerly uniting the two oceans, as has been so fre- 
quently represented. The ever lowering cols separating opposing 
drainages are cut out by the headwater erosion of the rivers them- 
selves in every instance where I could examine them. ‘This is 
certainly true of the Culebra saddle, the lowest pass in the whole 
Isthmian region. 
The present drowned condition of the mouths of these streams is of 
great interest, and attracts attention to their past history. The Chagres 
of the Caribbean coast, and Rio Grande of the Pacific side, whose head- 
waters ramify near the Culebra Pass, both reach base level far inland 
of the oceanic borders. The Chagres may be said to reach the marine 
base level at Gorgona, no rapids occurring below that point. At Bar- 
bacoas, 23 miles from the sea, it is only 40 feet above mean tide. In 
other words, the Chagres is a falling or cutting stream for only two 
thirds of its total length, the remaining one third of the distance being 
practically at marine base level. The Rio Grande of the Pacific side 
from Paraiso towards its mouth traverses the country for about nine 
miles, About three eighths of its distance, comprising the headwater 
portion, is a falling stream, the high tides of the Pacific backing up five 
eighths of its course. It is but a matter of time when the ultimate 
headwater erosion of the streams will make the long sought communi- 
cation between the oceans. 
The whole Land Surface has suffered general Lowering through 
Erosion. — What the ancient original configuration of the Isthmian 
region may have been is a matter of conjecture. It is no stretch of 
the imagination to say that the general level has been tremendously 
lowered by this long continued erosion, and that the former heights far 
exceeded the present altitude. The Isthmus of to-day, instead of being 
a new made land, is an old and decaying one, which has been dissolving 
away for ages under the effect of its tremendous rainfall, and is rapidly 
approaching base level. 
From the first sight of the Puerto Bello Islands across the Isthmus 
until sailing out into the Pacific beyond the rugged points of Cape Mala, 
the antiquity of this topography is impressed upon the observer. Old 
age is indelibly stamped upon every feature, and in the course of my 
