52 Cincinnati Society of Natural History. 
of the earth have not been subjected to such upheavals and de- 
pressions aS was once supposed; but that they have, instead, re- 
tained, in a remarkable degree, the same shape and general positions- 
they now have, for immense periods of time.* Especially must this 
be the case with America. In South America the immense alluvial’ 
deposits of the Amazonian valley, principally the result of the wear of 
ages on the masses of the Andes, have been formed since the last great 
change in the contour of the continent. In North America the alluvial 
deposits of the Mississippi valley, covering an area of 30,000 square — 
miles, in places 100 feet in depth, are supposed to have taken at least 
100,000 years to form,+ and the balance of the continent must be im- 
mensely more ancient. No other such deposits as these can be 
found in the world, and on this ground alone the immense antiquity — 
of the whole American continent can be maintained. , 
The fauna of North America points to the same conclusion. The 
remains of animals found in the Tertiary deposits of the Western 
United States indicate great antiquity, and the one family of horses 
alone would require a very long period of time to become differentiated 
into the many forms of which we now find remains. Among living 
forms is the large family of Humming birds, confined entirely to 
the American continent, and consisting of 118 distinct genera, and 400 
species. Mr. Wallace believes that this family originated in the | 
tropical forests of the once insular Andes, and has become differ- | 
entiated into so many distinct forms, because of the stability of the 
country, and the long continued existence of the present climatic con- 
ditions.} ' 
It can not, then, be asserted that we have not a lone period of time 
during which our plants could have been distributed, and there have 
been many causes which have helped to bring it about. I have else- 
where§ referred to certain agencies which are, or have been, very 
‘efficient in the work, and to one or two of them I wish to call your 
attention. 
No one now denies the fact that in times long past, the northern 
hemisphere down to the 40th degree of latitude in North America, 
and the 50th in Europe, was subjected to intense cold, and this period 
is known as the glacial epoch. Opinions differ as to the cause of the - 
cold, and many theories have been advanced to account forit. One of 
* Wallace, Tropical Nature, pp. 312, 313. 
+ Lyell, Antiquity of Man, p. 46. 
t Geog. Dist. of Animals, II., p. 322. See also Essay on Humming Birds in ‘* Tropical 
Nature,’’ p. 124, ez seg. Bi 
2 Popular Science Monthly, July, 1880. 
\ 4 : 
