Hershey.] 



Isthmus of Panama. 



237 



which are elevated above remnants of a low coastal plain which 

 once occupied the site of the gulf. 



On the west of the Aguadulce-Santiago plain, by the increase of 

 the monadnocks into a group of pointed hills occupying most of 

 the surface, a more elevated, broken country is produced, which 

 separates this plain from the plain of David in the province of 

 Chiriqui. Thus the former plain is surrounded on three sides by 

 mountainous tracts and is only open to the sea at its eastern end. 



Eastward from the head of the Bay of Parita, the Cordillera de 

 Veraguas, with its spurs, occupies the greater portion of the Isthmus 

 (the coastal plains being limited to narrow strips), until about 

 20 miles west of the line of the Panama railway and canal, where 

 the high sierra abruptly terminate and a different type of topog- 

 raphy prevails eastward. 



The topography of the country in the vicinity of the Panama 

 railroad between Panama and Colon has been so ably discussed by 

 Mr. Hill that I will avoid retraversing the ground except to make 

 a few general remarks. Like the preceding observer, I was strongly 

 impressed by the fact that there is not along this portion of the 

 Isthmus a well-defined central range such as geographers have 

 pictured as a connecting link between the Cordillera of North 

 America and the Andean chain of South America, but rather a 

 great group of peculiar pointed hills, extending from the islands in 

 the Gulf of Panama, two-thirds of the way across the Isthmus, and 

 which may be described, where not too closely grouped, as monad- 

 nocks rising from a low base-level developed on both sides of the 

 Isthmus but nowhere greatly elevated above the sea. I was also 

 impressed by the ancient type of the erosion topography and by 

 the great depth of weathering. It is beyond question that this 

 portion of the Isthmus has not been submerged at so late a period 

 as the Pleistocene, and, in fact, it has been a remarkably stable 

 portion of the continent since rather early in the Tertiary era. 



FORMATIONS EARLIER THAN THE- PLEISTOCENE. 



The Azuero Formation. — So far as now known, the oldest rock 

 on the Isthmus of Panama is a green eruptive formation. It is in 

 the form of a massif of unknown but immense thickness, which 



