HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 221 



(Plate VI. Fig. 2), and which he made at my request immediately after 

 my departure from Port Limon. 



Notwithstanding the contributions above mentioned I found several 

 new facts of interest concerning the geological history of this region, 

 among them the discovery of Cretaceous limestone near the summit of 

 the divide, the existence of granites on the eastern slope, and the 

 tracing of the details of the topographic evolution, — a subject to 

 which no attention had apparently previously been paid. 



The chief geologic features of the section across Costa Eica may be 

 divided into seven conspicuous categories, as follows : — 



1. The foundation rocks of ancient quartzites, serpentine, jadeite, and 

 granite, the latter rarely exposed, owing to concealment by the later 

 formations. These rocks are of unknown age, but probably Pre- 

 Cretaceous. 



2. Old limestones of supposedly Cretaceous age, outcropping at only 

 one small locality in the valley of San Jose over 5,000 feet above the 

 sea. 



3. The older basic igneous rocks, — mostly augite andesites and other 

 basic igneous rocks possibly of late Miocene and certainly of Eocene 

 Tertiary age, similar to those constituting the Isthmus of Panama. 

 These largely conceal most of the older formations, and are themselves 

 in turn largely buried beneath the volcanic accumulations of later 

 Tertiary and present time. 



4. The marine Tertiary sediments of the Caribbean side, from Eocene 

 to Pliocene age inclusive, which have largely been disturbed, elevated, 

 and broken through by igneous protrusions. 



5. The line of great volcanoes surmounting the crests of the Sierras, 

 and their accompanying igneous rocks. 



6. The Pleistocene or recent sediments of the coasts. 



7. The bolsons, base levelled plains, benches, canons, and other topo- 

 graphic features. 



The accompanying profile (Plate IV. Fig. 1, Plate V. Fig. 2, Plate VI. 

 Fig. 2) of Costa Rica shows the general character of the section which I 

 traversed, from ocean to ocean, and I shall not weary the reader with 

 more detail than necessary briefly to describe the chief features. The 

 profile is divisible into four well marked subsections. First, the penin- 

 sula of Xicoya, separated by the Gulf of Dulce from the mainland ; 

 second, the Pacific seaboard from Punta Arenas to the summit of the 

 Aguacate range of volcanic hills ; third, the central volcanic plateau, 

 extending from the Aguacate range to the eastern slope of Turialba ; 



