THEIR DEPOSITS AND DRAINAGE. 655 
less like the present than is that of the Miocene or Middle 
Tertiary. In this category of older deposits with a more 
tropical flora, I would place the Green River Tertiary beds, 
those of Mississippi studied by Lesquereux, and those of 
Brandon, Vermont. 
In tbe Miocene age, the continental surface was broader, 
the lake basins of the West contained only fresh water, and 
the land surface was covered with a vegetation very much 
like that of the present day; a number of Miocene’ species 
still existing. The climate of the continent in the Miocene 
age was much milder than now. Fan-palms then grew as 
far north as the Yellowstone River, and a flora flourished in 
Alaska and on Greenland as varied and as luxuriant as now 
grows along the fortieth parallel. At this time there must 
have been some sort of land connection between our conti- 
nent and Europe on the one hand and Asia on the other. 
The flora of all these regions was essentially the same, and a 
large number of plants were common to the three continents. 
In this age the mammalian fauna of our continent exhibited 
the same remarkable development that it did in Europe and 
Asia; and over our western plains roved herds of great 
quadrupeds rivalling in number and variety those that have 
Struck with wonder and surprise every traveller in South 
‘ica. 
— This state of things seems to have continued through the 
Pliocene age and up to the time when the climate of the 
continent was completely revolutioned by the advent of 
the “Ice period.” The change which took place at that time 
was such as taxes the imagination to conceive of, as much as 
it taxes the reasoning powers to explain. 
We have seen that in the Middle Tertiary age the climate 
of Alaska and Greenland was that of New York and St. 
Louis at present. In the next succeeding period, the glacial 
epoch, the present climate of Greenland was brought down 
to New York, and all the northern portion of the continent 
wrapped in ice and snow. This change was undoubtedly 
