HILL : GEOLOGY OF THE ISTHMUS OF PANAMA. 23o 



Vaughan, and are enumerated in his report appended herewith. They 

 are all common reef building species of the Caribbean region. 



Comparison of the Panama and Costa Rica Profiles. 



We have now presented two complete continental sections across con- 

 trasting portions of the greater Isthmian region. From Mr. Gabb's 

 unpublished paper * on the Province of Talamanca we give an additional 

 section of the Caribbean slope intermediate between Panama and San 

 Jose. The relations of the three sections are also clearly brought out 

 in the geological sections and profiles on Plate VI. 



The first and most striking contrast between the Panama and Costa 

 Rican sections is the different topographic aspect. The Panama section 

 is across an old land now nearly graded to the sea, where vulcanism 

 has been quiescent since Tertiary time. The Costa Rican section pre- 

 sents us a view of an ever growing laud where the volcanoes have con- 

 tinued to pile their debris from Cretaceous time to the present, suggesting 

 perhaps that on the Pacific side of the continent there has been a com- 

 pensating disappearance of vast lands by the sinking down of areas from 

 which this material has been extruded. 



In addition to the history of Tertiary sedimentation and vulcanism, 

 and the successive epochs of base levelling, the Costa Rican section gives 

 several interesting contributions to help round out the Central American 

 history. The presence of Cretaceous rocks, not found in the Isthmian 

 section gives us a view farther back into geologic history. The great 

 basin valley of the central plateau shows that at least as far back in 

 time as the Pleistocene there was extensive land with the diverse 

 topography of to-day. The marine Tertiary and later sediments of the 

 Caribbean coast of Costa Rica likewise both verify and amplify the rec- 

 ords of the Panama section. The presence of marine Pliocene sediments 

 resting unconformably against the folded older Tertiary rocks enables us 

 to fix more definitely the time of the late Miocene orogenic revolution. 



In this section we have the orogenic (fold-up) mountains of Creta- 

 ceous and Tertiary strata, the piled up ejecta of the volcanic cones, the 

 epeirogenic (lifted-up) coast benches, and the coral reefs of the Atlantic 

 coast. The benches and base levelled plains of the Costa Rican coast 

 record epeirogenic movements of late geologic time in harmony with 

 those of the Panama coast. 



The general conclusions to be drawn from these continental sections 

 may be tabulated as follows. 



1 In the library of the United States Geological Survey. 



