HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 265 



The Culebra clay of supposedly Eocene age, which we have hypothet- 

 ically correlated with the great coal formation of the Caribbean side of 

 the Isthmian region, is the only sedimentary formation which occurs near 

 the drainage divide of the oceans, and this is found only on the Carib- 

 bean side. The composition, structure, and fossils of these clays give 

 no evidence of their deposition in a passage connecting the oceans, espe- 

 cially a deep and extensive one. These clays are apparently entirely 

 void of molluscan remains such as surely would have inhabited such a 

 passage. In fact, their composition, made up so largely of laud debris 

 and plants, and their extensive distribution, are, in my opinion, capa- 

 ble of but one interpretation, that, instead of indicating a free marine 

 passage, they attest the existence of larger surrounding land areas in 

 this region than now exist. 



Although the biologic and paleontologic evidence leads to the conclu- 

 sion that a shallow connection did exist somewhere in Tropical America 

 during Eocene time, neither the geologic nor the biological evidence 

 shows conclusively that the Isthmus of Panama was the exact place of 

 this connection. It merely proves that such a connection was probable 

 somewhere in that vast Tropical region betweeu the known localities 

 of the Tejon beds in Lower California and the place of the occurrence 

 of the Gatun beds on the Caribbean side near Colon. 



The occurrence in the Tertiary of a few species common to the Atlan- 

 tic and the Pacific have been reported by G. B. Harris as far northward 

 as Galveston, Texas. A connection of the two oceans in Tertiary time 

 in that latitude, from the well known evidence of the geological forma- 

 tion and history, would be preposterous. 



The Oligocene and later faunas of the continental coast succeeding 

 the Claiborne, so far as the writer is aware, are thoroughly Caribbean, and 

 show no evidence of free commingling of species of the two oceans. 1 



Harmony of Geologic Evidence with Biologic and Paleontologic Deduc- 

 tions that Land Connection was thoroughly established during the Close 

 of the Miocene. — If the marine passage ever existed across the Isthmus 

 or elsewhere in Tropical America, it must have been during the later 

 Eocene epoch, for all evidence concerning later epochs shows that the 

 great land barrier connecting the continents has existed since that time, 

 and that the seas have never since communicated across it. Hence the 



1 It has been alleged by several writers upon paleontology that Pacific forms — 

 especially corals — do exist in the Miocene (Late Oligocene) fauna of the Great 

 Antilles. We have proofs that many of the specific correlations were erroneous. 

 This subject will be discussed in our report on Jamaica. 



