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University of California. 



[Vol. 2. 



The plain everywhere bevels the edges of the slightly upturned 

 strata. 



Toward Aguadulce there are broad stretches of open grassy 

 plain which are remarkably even but not flat; that is, they are 

 characterized by long, extremely gentle even slopes which form 

 shallow basins, in the deepest portions of which a few streams may 

 be found in the rainy season. Farther west toward Santiago, the 

 plain becomes more rolling and monadnocks more numerous. 

 Because of an uplift the streams do not now flow on the peneplain, 

 but have eroded beneath its surface valleys to as much as 100 feet 

 in depth. On the inter-fluvial portions the original peneplain sur- 

 face remains as grassy llanos, very slightly arched in the center 

 and remarkable for their long, slight, even slopes. Low ranges of 

 hills (residuals) traverse the plain in various directions aid often 

 isolated, rounded elevations are encountered. 



Standing on the foot-hills of the cordillera and looking over the 

 plain, it is seen to be a perfectly base-leveled area of denudation. 

 The shallow basins, the low divides, and the canon valleys disap- 

 pear with distance and we see only a land apparently as flat as the 

 ocean except for the many monadnocks which rise sharply from it. 

 It is the most beautiful and perfectly base-leveled land with which I 

 am acquainted. 



The Pleistocene base-level is found at intervals along both 

 coasts of the Isthmus and was recognized by Mr. Hill at Colon, in 

 the truncated summits of the Monkey Hills, and at Panama, 

 where it constitutes the terrace on which the city is mainly built. 

 Mt. Aucon, which rises prominently just beyond the city, and most 

 of the islands in the bay, were monadnocks or residuals on this 

 peneplain. It was once developed over the area of the Gulf of 

 Panama, but within its limits is now represented by a single rem- 

 nant, a flat-topped island about 10 miles southwest of the city. 



At the foot of the Cordillera de Veraguas on the northern or 

 Caribbean side of the Isthmus, base-leveling was effected on the 

 hard crystallines and andesitef?) of the Veraguas series over a belt 

 at least several miles wide. This has since been uplifted and tilted 

 toward the sea. The streams have eroded in the plain narrow 

 carion valleys from one to several hundred feet deep, but all the 



