HILL : GEOLOGY OF THE ISTHMUS OF PANAMA. 1G7 



tirely of altitude, but also of age and structure. This plateau has a 

 foundation of Cretaceous (and in Guatemala earlier sedimentaries) no- 

 where as yet positively found in the Panama Isthmus. Upon these, in 

 places, are Tertiary sediments similar to those of the Isthmus. These in 

 turn have been buried beneath the thousands of feet of debris from the 

 volcanoes which here have continued active from the Tertiary period to 

 the present, during which time the Isthmian land has beeu undergoing 

 a general lowering through erosion. 



Topographic Evidence bearing upon the Antiquity of the 

 Isthmian Region. 



I have represented two parallel adjacent profiles of the Isthmus. 

 (See Plate IV.) One of these is along the line of the canal and railway 

 which follows the lowest passes of the drainage, and conveys an erro- 

 neous impression that the country consists of a simple anticlinal slope 

 separated by a continental divide. 



The other (Plate IV. Fig. 2) is run in a straight line across the 

 same section, commencing on the west side of Limon Bay, and pass- 

 ing through the Cerro Ancon at the city of Panama and across the Gulf. 

 This profile shows the topography of the country in its natural aspect 

 and relations. It will be seen that, instead of presenting the two well 

 defined coastal slopes and a single continental divide, the country is 

 one of great irregularity, showing no such features. Its hills, called 

 by courtesy mountains, rise precipitously almost directly from the level 

 of each ocean, and occur completely across the Isthmus into the Gulf of 

 Panama, regardless of a continental comb. The whole topography of 

 the region indicates that it is an area of irregular summits arranged 

 without the presence of a single central ridge. The Culebra summit, 

 so called, instead of being the highest point of the mountainous topog- 

 raphy representing a continental backbone divide, is excelled in height 

 by many eminences. At Colon the hills on the west side of Limon 

 Bay of the Atlantic shore are almost as lofty as the Culebra summit, 

 while at Gorgona some of the volcanic hills not over a mile south of the 

 road are over 1,000 feet in height. Other eminences a few miles away 

 from the railroad, eastward toward Puerto Bello and in the region of 

 Chorrera, rise to a height of 1,000 and 1,500 feet. In the light of this 

 profile the Culebra summit, as far as it alludes to the continental divide, 

 is a misnomer and misleading. The pass is nothing more or less than 

 an eroded drainage saddle, which has been and is constantly being 

 lowered by the headwaters of the Obispo and Pdo Grande. Even the 



