256 BULLETIN: MUSEUM OF COMPAEATIVE 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 the 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, and 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 of 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 



