Hershey.] 



Isthmus of Panama. 



^55 



Over the Aguadulce-Santiago plain the uplift has been slight 

 (50 to 500 feet) and the strata soft ; hence, erosion here has pro- 

 ceeded to such an extent as to leave the original peneplain surface 

 remaining only in widely separated tracts of exceedingly limited 

 area. In the foot-hill region, the erosion of valleys from 1,000 to 

 2,500 feet in depth has carried away fully two-thirds of the strata 

 between their bottoms and the old peneplain. In the cordilleran 

 region, the old plateau is nearly thoroughly dissected by valleys 

 several thousand feet in depth and of no mean width. The val- 

 leys in this Cordillera de Veraguas may be directly compared with 

 those of the Black Hills of South Dakota, the deep canons of the 

 Sierra Nevada and Klamath Mountains of California, and certain 

 deep but not broad valleys of the Appalachian region, as those of 

 the West Virginia plateau. Taking into consideration the hard 

 crystalline rock, but otherwise favorable conditions for rapid 

 erosion, the valleys of the cordillera need not have required such a 

 long period of erosion as the first and last of those in the United 

 States mentioned above, although it is hardly possible that they 

 could have been excavated solely within the Pleistocene era. I 

 believe the evidence warrants our provisionally classing the uplift 

 of the peneplain as an event of the Pliocene period. 



The late Tertiary uplift was of the nature of a broad arching of 

 the rocks with an east-west axis occupying the position of the 

 present high sierra. The peneplain in the cordilleran region was 

 not immediately raised to its present altitude of about 5,000 feet, 

 for part of the uplift is due to movements as recent as the late 

 Pleistocene. 



The Pleistocene Peneplain. — When I first traveled over the beau- 

 tiful grassy plain with island-like clumps of dark green arborescent 

 vegetation, between Aguadulce and Santiago, I assumed without 

 serious questioning that it is an old coastal plain of aggradation, 

 and the isolated, often cone-shaped elevations on it, volcanic cones. 

 But when I came to study the strata under it, I found, instead of 

 the loose sands and clays of Pleistocene age that I had expected, 

 conglomerates, breccias, shales and tuffs of Eocene and earlier age, 

 the whole horizontal in a general way, but considered in detail 

 always dipping perceptibly and often decidedly in some direction. 



