208 bulletin: museum of comparative zoology. 



tion vulcanism was active in the Isthmian region prior to or during 

 this epoch. 



An important change in the sedimentation of the Tertiary beds must 

 now be noted. The impure character of the Vamos a Vamos beds sud- 

 denly changes into an extensive uniform formation of greensand marl 

 of the Mindi Hill beds, as is shown by the exposures in the Mindi, 

 Monkey Hill, and Derivation cuts. Dr. Dall naturally thinks that this 

 change of sediment indicates that these later beds are the more finely 

 triturated material deposited in slightly deeper waters, but inasmuch as 

 the fossils are the same as those of the Vamos a Vamos formation, I am 

 inclined to believe that the difference is due to the fact that the Mindi 

 sediments no longer received in their composition the volcanic material 

 which was freely intermingled with the lower lying Vamos a Vamos 

 beds during a contemporaneous active igneous epoch. In other words, 

 this change gives a clue as to the time of the cessation of the Eocene 

 igneous activity. 



The Monkey Hill beds, which lithologically resemble the Mindi beds 

 and apparently belong to the same cycle of sedimentation, have a fauna 

 which shows that they are of later age, being equivalent, according to 

 Dall, to a lower Miocene (Upper Oligocene) fauna which has wide dis- 

 tribution in the Caribbean region, such as are found in Curac,oa, Haiti, 

 the Bowden beds of Jamaica, and the Chipola beds of Florida. 



The foregoing mentioned Isthmian beds, so far as known, constitute a 

 great and continuous epoch of sedimentation, embracing the Eocene and 

 Oligocene epochs. 



One of the most instructive points in the Tertiary section is its abrupt 

 termination with the early Miocene, indicating a great geomorphio 

 change, following that period. No trace is preserved to indicate that 

 sedimentation occurred on the area of the present Isthmian region during 

 the late Miocene and Pliocene epoch, and the only legitimate inference, 

 fortified by other lines of evidence, is that the Isthmian land was of 

 much larger area during these later epochs than in Eocene time or at 

 present. 



The thickness of the Tertiary rocks in the Caribbean section is diffi- 

 cult to estimate. At least six hundred feet of outcrop is directly ex- 

 posed in the various hills, but it is impossible to estimate the thickness of 

 that which is concealed. A thousand feet would represent the minimum 

 approximate thickness, but it may have been double or tenfold this. 



The fossils of the late sedimentation of the swamp levels of both sides 

 of the Isthmus show that these beds are marine littorals of late Pleisto- 



