HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 235 
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- 
irasting 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unpublished paper!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 
José. Тһе 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 topographie aspect. The Panama section 
is across an old land now nearly graded to the sea, where vulcanism 
has been quiescent since Tertiary time. Тһе Costa Rican section pre- 
sents us a view of an ever growing land where the volcanoes have con- 
tinued to pile their débris 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. Тһе 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 orogenie (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. 
