250 GEOLOGICAL CHANGES 3NN0ETHERN AND CENTRAL ASIA. [MayiQOi, 



the surface is here 630 feet above the sea. The Dnieper has cut a 

 trough in this plain to a depth of 350 feet, exposing the strata in a 

 very convenient manner. The loess constitutes the upper 50 feet 

 of the deposits. The stone-implements were found below the loess, 

 at a total depth of 53 feet. Thus it appears that the continental 

 submergence which aided in the wide distribution of the loess was 

 subsequent to the appearance of man, and so another chapter is 

 added to those which connect the ancient history of the human 

 race with the more recent phases of the geological story. 



If, in conclusion, we are permitted to speculate upon the causes of 

 the contrast between the post-Tertiary conditions of Asia and those 

 of North America, we may assume that the absence of continental 

 glaciers in Asia may be partly due to those permanent climatic 

 conditions which render so much of Central Asia almost rainless, and 

 all of Northern Asia a comparatively dry region. North of the 40th 

 parallel the annual precipitation is everywhere less than 20 inches 

 in Asia, except over a comparatively narrow belt along the sea-coast 

 in the extreme east. But I am inclined to think that difference of 

 elevation may have been the cause. In America there is abundant 

 evidence that the Glacial Period was coincident with a continental 

 elevation of 2000 or 3000 feet. If, as seems likely, the depression 

 of the land in Asia was coincident with tlie elevation in America, this 

 may well be the cause of the absence of glaciers which we have 

 found to characterize Northern Asia during post-Tertiary times. 



Discussion. 



Mr. F. B. Wright, Jr., and Mr. G. W. Lamplfgh spoke. 



The Author remarked that fossils of any sort were very rare in the 

 loess everywhere. In America the land-shells found in it indicated 

 Tnoister conditions than those now prevailing ; but, as water freely 

 percolates through the loess, it is likely that by this means most 

 fossils would be dissolved. This may be the cause of the numerous 

 calcareous concretions which abound in the formation. It is fairly 

 certain that the loess in America was distributed by water-action : 

 the bluffs of loess, for example, at Vicksburg, in the middle of 

 the Mississippi Valley, could hardly have been deposited by wind. 



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