No. 388.]. REVIEWS OF RECENT LITERATURE. 351 
The lowest pass across the isthmus, which is a drainage col between 
the head waters of two opposite flowing streams, is but 287 to 295 
feet above the ocean. If the isthmus should subside about 300 feet, 
a connection would be made between the waters of the Atlantic and 
the Pacific. None of the passes through the mountains, however, 
give evidences of ever having been straits connecting the bordering 
oceans. One of the most important conclusions reached by Hill 
from several lines of investigation, is that there has been no oceanic 
connection across the isthmus since Tertiary time. There is, indeed, 
considerable evidence that this land barrier has existed since Jura- 
Trias time, with perhaps a shallow passageway across at the close 
of the Eocene; but even this ephemeral connection has not been 
definitely proven. In a footnote the opinion is expressed that the 
Tehuantepec isthmus, composed of cretaceous rocks, has “ remained 
land since its earliest origin.” 
The conclusion in this connection seems to be that the Central 
American and Mexican region has been above water at least since 
the close of the Tertiary. This is of special importance to the 
students of the Glacial epoch, inasmuch as a subsidence of a portion 
at least of the Central American region, and'a consequent cessation 
of the Gulf Stream, has been postulated, on purely hypothetical 
grounds, to explain climatic changes in the northern portions of 
America and Europe. The antiquity of Central America is also of 
much interest to biologists, since it is in harmony with the well-known 
differences in most of the marine species in the waters it separates. 
Much information is recorded by Hill in reference to the rocks of 
the isthmus, the geological structure, the decay of the surface mate- 
rial under a warm climate with excessive rainfall, etc. The appen- 
dices contain the following reports on the collections brought home: 
invertebrate fossils, by W. H. Dall; foraminiferal deposits, by R. 
M. Bagg ; and igneous rocks, by J. E. Wolff. 
The report is illustrated by three instructive sketch-maps in con- 
tours, which unfortunately are without titles or scales, four sheets of 
profiles and sections, and twelve reproductions of photographs, several 
of which illustrate scenes along the Panama Canal. 
ISRAEL C. RUSSELL. 
