270 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



at 186 feet. Dr. J. W. Gregory 1 states "that the summit of the 

 Isthmus at one locality is 154 feet, and in another 287 feet in height." 

 The lowest Isthmian pass which is not a summit, hut a draiuage col, is 

 287-295 feet above the ocean. 2 



If we could lower the Isthmian region 300 feet at present, the waters 

 of the two oceans would certainly commingle through the narrow Cule- 

 bra Pass. But the Culebra Pass is clearly the headwater col of two 

 streams, the Obispo flowing into the Chagres, and the Rio Grande flow- 

 ing into the Pacific, and has been cut by fluviatile action, and not by 

 marine erosion, out of a land mass which has existed since Miocene time. 

 Those who attempt to establish Pleistocene inter-oceanic channels through 

 this pass on account of its present low altitude, must not omit from 

 their calculations the restoration of former rock masses which have been 

 removed by the general levelling of the surface by erosion. 



Summary. 



There is considerable evidence that a land barrier in the Tropical 

 region separated the two oceans as far back in geologic history as Juras- 

 sic time, and that that barrier continued throughout the Cretaceous 

 period. The geologic structure of the Isthmus and Central American 

 regions, so far as investigated, when considered aside from the paleon- 

 tology, presents no evidence by which the former existence of a free 

 communication of oceanic waters across the present tropical land barriers 

 can be established. The paleontologic evidence indicates the ephemeral 

 existence of a passage at the close of the Eocene period. 



All lines of inquiry — geologic, paleontologic, and biologic — give evi- 

 dence that no connection has existed between the two oceans since the 

 close of the Oligocene. This structural geology is decidedly opposed to 

 any hypothesis by which the waters of the two oceans could have been 

 connected across the regions in Miocene, Pliocene, Pleistoeene, or recent 

 time. 



1 Quart. Journal Geol. Society of London, Vol. LI., Part 2, No. 203, page 299, 

 1895. 



2 The height in feet of the natural passes of the Isthmus of Panama, according 

 to known engineering data, are as follows : Culebra, 287-295 ; Atrato-Sucubti, 583 ; 

 Atrata-Napipi, 778; Caledonia Pass, 1,003; San Bias, 1,142 ; Atrata-Morte, 1,143. 

 The Nicaragua Pass is 147 feet ; Tehuantepec, 858 ; Honduras, 2,956. 



