Cope.] 100 [April 7, 
fishes, then, passed through fresh water connections, existing on a conti- 
nent now submerged beneath the Pacific Ocean, seems probable. 
The destruction of the Pliocene fauna is generally admitted to have 
been brought about by the rigors of the glacial climate, and the extension 
southward of the ice sheet and snow falls. Near the same time, con- 
nection with Asia must have been severed by the descent of the North 
Pacific Continent. Some Pliocene types, not now existent in North 
America, may have been driven into the Neotropical region, and may be 
still represented in their descendants, the Lamas, the only existing Cu- 
melide of the new world, with the horses and perhaps others of the higher 
mammalia of that region, The existence of the extinct Mastodon, Mach- 
aerodus, etc., in the postpliocene of the same region, mentioned by Hux- 
ley, as a puzzling fact, (Address l. c.) may be accounted for in-the same 
way. 
Of course, on the northward retreat of the ice sheet, the mammalia 
fauna would have to be derived from the south, for communication direct 
with Asia no longer existed. If Behrings straits were not yet opened, 
the masses of glacial ice covering those regions would effectually prevent 
immigration by that supposed connection. The resulting Postpliocene 
fauna would naturally partake of the mixed character which our brief in- 
vestigations into it have revealed. The neotropical forms would occupy 
regions left vacant, or peopled by a sparse remnant of boreal genera and 
species. This view I proposed some time ago,* and Dr. Leidy has added 
his valuable opinion to the same effect.+ 
Has any great disturbance of level intervened between the occupation 
of the post-pliocene fauna and the present period? Prof. Dana (Manual 
of Geology, 1862,) summarizes the results attained up to his writing (p. 
553), by showing that the period succeeding the glacial drift was one of 
submergence, especially in arctic latitudes. He states the depression near 
Montreal to have been 450 or more feet, and 1000 feet in Arctic regions. 
Of the Middle States he says nothing, and of the south, that the evidence 
is not satisfactory. This descent of level he regards as that which caused 
the melting of the glacial ice, stratification of the drift, deposition of 
gravels, and elevation of temperature. All these changes would natu- 
rally precede the introduction of a postglacial fauna from a warmer 
region, so that for this and other reasons, the Champlain epoch may be 
regarded as that opening the post-pliocene, and its fauna to be repre- 
sented by the Walrus, which extended its range to Virginia, the Reindeer 
to New Jersey, andthe Beluga of the Champlain clays. 
The origin of the caves which so abound in the limestones of the Alle- 
gheny and Mississippi valley regions, is a subject of much interest. Their 
galleries measure many thousands of miles, and their number is legion. The 
writer has examined twenty-five, in more or less detail, in Virginia and 
Tennessee, and can add his testimony to the belief that they have been 
formed by currents of running water. ‘They generally extend in a direc- 
*Proceed, Acad. Nat. Sci. 1867, 156, 
+Mammalia Dakota and Nebraska, 359, 1869, 
