HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 169 



and actively working headwaters. . A 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, Bay an o, 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 Eio 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 



