GLACIAL EPOCH UPON THE EARLY HISTORY OE MANKIND. 61 
the Glacial Epocli probably continued far down towards its close, 
when it was followed by a depression to a level considerably 
below that of the present time. This is clearly evinced by 
post-glacial marine deposits and beaches which are now found 
several hundred feet above the sea in Canada and Scandinavia. 
The extreme depression, as shown by these raised beaches, was 
in both places fully 1,000 feet. It is evident also that, at the 
time of the extreme extension of glacial ice in America, the 
gradient of all the south-flowing streams was greatly reduced, 
indicating a differential northerly depression over the whole 
interior of the United States. 
It is easy to see, therefore, that during the culminating 
period of the Glacial Epoch, man and his contemporary animals 
in America were shut off from communication with Asia, and 
the area from which they derived subsistence was greatly 
limited, both by the submergence of the continental shelf and 
by the great extension of the ice fields, reaching in the 
Mississippi Yalley in Southern Illinois the latitude of 38°. 
At the same time, there was a great incursion of the ice upon 
the fertile portions of Europe. Switzerland was obliterated, 
Great Britain nearly so, all Northern Germany was covered, and 
Bussia to within a short distance of the Black Sea. 
On the other hand, Central Asia seemed to receive a great 
increase of fertility. From recent investigations it appears that 
Siberia and Central Asia were not invaded by glacial ice.* But 
there was a great extension of the glaciers still existing in the 
high mountains. Those of the Thianshau range merit special 
mention. This vast mountain system rises in peaks to a height 
of 23,500 feet, or 8,000 feet higher than the Alps ; while its 
mass is estimated to be twenty times that of the Alps. Small 
glaciers still exist far up in the higher altitudes. During the 
Glacial Epoch they descended to the 7,000 foot level, but never 
readied the great plains at a lower level. A subsidiary result 
of this extension of the mountain glaciers in Central Asia was 
a marked increase in the size of the mountain streams upon 
which the population of the plains depended for irrigation. 
The importance of irrigation to the population of Central 
Asia is not generally appreciated. Our attention has so long 
been fixed upon Egypt and its dependence upon the Nile that 
we have not given sufficient consideration to other regions 
dependent upon irrigation. Now, around the base of the 
Thianshan mountains, there is an area many times the size of 
* See Wright’s Asiatic Russia, 2 vois., McClure & Co., New York. 
