214 bulletin: museum of comparative zoology. 



ting out of the greater stream valleys and the creation of the Monkey 

 Hill and Panama base levels, which then marked the sea margin. The 

 now drowned topography of Panama Bay is probably a remnant of this 

 old erosion epoch. 



The Monkey Hill and Panama benches on opposite sides of the 

 Isthmus are old base levelled plains of erosion, which, near the close 

 of the Miocene, as closely as can be approximated, were at the level of 

 the sea, and were partially elevated to their present height during the 

 succeeding epochs, for their continuity has been cut through by 

 the Pre-Pleistocene erosion valleys. The epoch of late Miocene and 

 Pliocene erosion was followed by a period of slight subsidence in Pleisto- 

 cene time. 



The marine swamp-level formations clearly occupy the old (Pliocene) 

 valleys eroded out of the late Tertiary topography, which through 

 subsidence were invaded by the sea during the epoch of their depo- 

 sition, but it is positively shown that these valleys did not trans- 

 gress the continental barrier. There can be little doubt that the 

 great Panama Gulf was largely submerged during this epoch of sub- 

 sidence. A general lowering of the land after a period of extensive 

 base levelling is the only method by which this peculiar body of water 

 ■can be accounted for. 



The present position of the contemporaneous Pleistocene sedimen- 

 taries of the swamp-level beds on both sides of the Isthmus show that 

 since Pleistocene, or even during recent time, an epeirogenic uplift or 

 uplifts have elevated the whole region at least ten feet above their 

 previous level of deposition. This elevation of the swamp levels above 

 the ocean in Post-Pleistocene time is the last epoch of Isthmian history 

 recorded in this section. 



But in spite of much information obtained upon many hitherto ob- 

 scured points we are unable to give a complete interpretation of the 

 history of the Isthmian region. From the fossils we can only see as 

 far back ha geologic time as the Eocene, and these point out only one 

 brief epoch when there could have been connection, although there are 

 non-fossiliferous formations antedating that age. The structure, so far 

 as known, and when considered alone, gives no information as to the 

 period of geologic time when the waters of the two oceans connected 

 across the isthmus. In the Panamic section I found no continuity of 

 beds from the Caribbean to the Pacific side, which would positively 

 prove the former existence of a free oceanic strait. The Empire lime- 

 stones, the great accumulation of boulder conglomerates, the foraminif- 





