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. Тһе 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 lato 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 
baso 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 opeirogenie 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 
che 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 in geologio 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- 
