270 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
at 186 feet. Dr. J. W. Gregory! 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, but a drainage col, is 
287-295 feet above the ocean.? 
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. Тһе geologie 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. Тһе paleontologic evidence indicates the ephemeral 
existence of a. passage at the close of the Eocene period. 
САП 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, Pleistocene, 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, 588 ; 
Atrata-Napipi, 778; Caledonia Pass, 1,008; San Blas, 1,142; Atrata-Morte, 1,143. 
The Nicaragua Pass is 147 feet; ‘Tehuantepec, 858; Honduras, 2,956. 
