256 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
the close of the mountain making epoch of late Miocene time, has 
participated in certain regional movements by which its whole area has 
been alternately elevated and depressed, but the amplitude of these 
phenomena is such that they cannot have produced any condition by 
which the Isthmian barrier was destroyed and the waters of (һе two 
oceans between them united. 
From the similarity of these base levellings, valley forms, altitude of 
marine deposits, and resemblance of aggradational formations on both 
the Atlantic and Pacific sides, there can be no doubt that these events 
took place after the paroxysmal period of Isthmian deformation, and 
represent the etching and gradation on opposite sides of a great land 
barrier between the oceans which then existed, as to-day, amd there 
can be no doubt from the present position of both the Pliocene and 
Pleistocene, deposited unconformably against the older Tertiary forma- 
tions, that the Central American and Isthmian land masses were well 
defined before these later epochs. 
The low passes across the summits of the Isthmus are not marine, 
but are solely and simply the work of the downward erosion of the 
headwater streams of the opposing drainage systems, and this lowering 
is still in progress. The lowest known pass across the drainage divide 
of the Isthmus is that of the Culebra, along the line of our section, and, 
while the waters of the ocean approached this divide much nearer than 
to-day, the amplitude of the subsidences was never sufficient to have 
submerged it. 
The last event in the Isthmian history was the widely extended 
epeirogenic movement which resulted in the slight elevation of Post- 
Pleistocene time, — a movement which is traceable from New England 
to the mouth of the Orinoco, and which was participated in by the 
Antilles, resulting in well known phenomena of all these regions. 
The events of the periods of time embraced in the inseparable Pliocene, 
Pleistocene, and recent epochs were undoubtedly accompanied by three 
results : — 
1. Extensive erosion and base levelling of the newly heightened land 
following the orogenic revolution of late Tertiary time. During this 
earlier epoch the great valleys оҒ the Tuyra, Atrato, and Chagres were 
probably largely made. 'The Monkey Hill and Panama benches prob- 
ably represent the marginal plains of this erosion period, which may 
be assigned to late Pliocene time. 
2. Subsidence (epeirogenic lowering), accompanied by the conversion 
of the previously made erosion valleys into the tide water estuaries, and 
