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Bibliographical Notice. 493 
ceros (three species), and a great variety of ruminants, rodents, 
carnivores, &c., forming more than sixty species, many o of 
re size. Such a groaping of animals might be considered indica- 
ve of a warmer climate than the present; but the associated plants 
bieten some living species, and are all closely allied to our present 
flora. Of the freshwater shells found in these beds the same may 
the Mississippi, from strata which c rrespond v very closely in age 
with ge containing the Plioceno vethebtalnn, include a still larger 
number of living species. 
“We co e every reason to believe, therefore, that during the 
cae and Pliocene Tertiary periods the form and climate of our 
ontinent were similar to what they now are, that, while the cli- 
across the continent very much as now. is indicated by the 
flora on the north-west coast, er includes several of Dr. Hayden’s 
Upper-Missouri species, and yet with them some which he did not 
find, and such as are indicative te a peo es warmer climate 
“ The Drift Period.—Having such an approach to the present con- 
dition of our continent during the later Tertiary ages, when many of 
our living species of both animals and plants were introduced, we 
have been very slow to accept the evidence which the glacial epoch 
ry, scar 
Briones! history. It would be foreign to my purpose to discuss 
e all the phenomena of the Drift Period; and it will be sufficient 
ios give conclusions without the proofs upon which they rest, merely 
remarking that these conclusions are now generally accepted: by 
those who have bad the best opportunity of observing the facts. 
They are in general terms as follows :— 
“First. That during the Drift Period a great depression of tem- 
perature took place over our continent, and that, at a co fesponding 
period i in the chain of events (whether synchronous or not), a similar 
cold period occurred in the Old World. 
“Second. That during the prevalence of this cold term ice co- 
vered the greater part of the hemisphere down to the parallel of 38° 
or 40°, and covered the summits of our western mountain-ranges 
down to the northern line of Mexico and Arizona. South of these 
limits we have no proof of the exi^tence of glac ers; and if they had 
existed, they would have been sure to leave their traces behi 
them. During this cold interval the fauna and flora of the northern 
po-tion of oar continent must have been driven southward many 
degrees below their previous and present range. 
“E After a longer or shorter period of continuance, the 
M Fo urth. ben e melting of the aceumulated snow ‘aa ice, the 
central portions of the continent were flooded, and the basin of the 
Ann. & Mag. N. Hist. Ser. 4. Vol. vi. 33 
