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University of California. 



LYol. 2. 



material, as they passed from one basin to another. No relics of 

 this necessarily unsystematic drainage system are left. In other 

 words, in none of the streams of the Isthmus can the determination 

 of the course be traced back to this volcanic period. Indeed, the 

 volcanic cones and craters themselves have all disappeared (except 

 possibly the recent volcano of Chiriqui), and all the medium and 

 minor features of the topography are due to sub-aerial erosion. 



The Tertiary Dissected Peneplain. — Soon after I first recognized 

 the cone-shaped elevations and low isolated hill ranges on the 

 Aguadulce-Santiago plain as monadnocks on a Pleistocene pene- 

 plain, I noticed that while they vary in height between 50 and 500 

 feet, nearly all of those in any given section of the plain attain 

 about the same elevation. Many of the hill ranges have com- 

 paratively even crest-lines, and a few are flat-topped as though 

 they are remnants of a plain. Indeed, the summits of practically 

 all the monadnocks on the plain will fall naturally in a slightly de- 

 formed, nearly destroyed plain. At first I thought this was a 

 constructional plain or the original surface at the close of the 

 volcanic period; but I found that the monadnocks are composed of 

 a great variety of formations, among which I may mention Santiago 

 shale, Eocene red clay, the tuffs of the Panama series and the dikes 

 and volcanic necks of the same. Besides it is impossible that the 

 original surface in such a volcanic region could have been so perfect 

 a plain as is indicated by the summits of the monadnocks. It is 

 quite evident that there had here been some erosion, and the man- 

 ner in which the supposed plain of the summit level of the hills 

 beveled the edges of the inclined strata suggests peneplanation. 



Transferring our attention to the top of the high sierra which 

 bound the view on the north, we find that the Cordillera de Vera- 

 guas appears from a distance as an extremely abrupt single ridge, 

 somewhat broken in places by valleys eroded in its flanks, and 

 having a comparatively uniform altitude of about 5,000 feet. But 

 when we travel about within its limits, we find that it is in reality 

 only a deeply dissected plateau. All the adjacent ridges rise to about 

 the same height and often have even crest-lines for several miles. 

 There are some extended flats at the level of the mountain summits, 

 as the high llano on which flows the. upper stretches of the Rio de 



