Hershey.] 



Isthmus of Panama. 



251 



The new folds had an east-west trend and the Cordillera was built 

 across the ends of the old north-south system. 



We are now ready to formulate a great principle in the geolog- 

 ical history of the Isthmus. From the earliest times of which we 

 have any record to the present day, there has been a tendency 

 toward a southward tilting of the entire country. While a great 

 highland belt was being added to the Isthmian country on the north 

 side of the Aguadulce-Santiago plain and its original far eastward 

 extension, the "old land" area on the south was being destroyed, 

 partly by the action of the sea along the shore and partly by sub- 

 sidence. The whole of the presumably pre-Cretaceous land area of 

 this region has gone beneath the Pacific's waters, except the south- 

 ern part of the Peninsula of Azuero, and perhaps a few small areas 

 west from the Gulf of Montijo. Eastward from the Peninsula of 

 Azuero, the sea has swept over the "old land" and the plains belt 

 north of it and in the vicinity of Panama has even invaded the 

 volcanic highland. 



DEVELOPMENT OF PRESENT TOPOGRAPHY. 



Under this heading it is proposed to outline the chief physio- 

 grapic events of the later geologic history of that portion of the 

 Isthmus lying west of the Panama Railroad and east of the province 

 of Chiriqui. The description of the various Pleistocene formations 

 has been reserved for this section of the paper, as their history is 

 inter-related with that of the geomorphogeny of the land surface. 



The earliest date with which we are now concerned is the close 

 of the great Panama epoch of volcanic activity. As the Panama 

 series is made up of a vast thickness of consolidated tuffs, with 

 intruded and intercallated sheets, irregular masses, trap dikes and 

 volcanic necks of diorite, andesite, basalt, etc., it may be presumed 

 that the surface at the close of the period was a very uneven one, 

 with a type of topography similar to that so beautifully exemplified 

 in the recent volcanic ranges in Guatemala and Nicaragua; that is, 

 it abounded in cone-shaped peaks and craters, with variously- 

 shaped basins between the individual volcanoes. Through this 

 land of heterogeneous topography the streams were forced to flow 

 hither and yon by the accidents of unequal deposition of ejected 



