THE 
AMERICAN NATURALIST. 
Vou. xv. — FANUARY, 1881. — No. 1. 
THE ANCIENT GLACIERS OF THE ROCKY MOUNT- 
AINS. , 
BY ARCHIBALD GEIKIE, F.R.S.! 
Lo the many profoundly interesting questions in Ameri- 
can Quaternary geology, one of the most important is un- 
questionably the determination of the area and movements of the 
ice during the glacial period. In the Eastern States much has 
now been done towards the elucidation of this problem. The 
general southern limits of the great ice sheet, and its course from” 
Canada southwards have been more or less definitely fixed; 
though much remains to.be done before our knowledge even on 
these points can be regarded as more than a mere outline of the 
truth. When, however, we try to realize the relations of the ice- 
sheet westward of the Mississippi basin, we soon perceive the 
meagreness of our information in regard to these Vast western 
regions. But it is there that one of the most remarkable parts of 
the glacial problem must be solved. The important labors of 
Dr. Percival and Profs. Whitney, Irving and Chamberlain, have : 
shown that even within the area embraced by the northern ice- | 
sheet, there was a tract of about 12,000 square miles in Wiscon- =. 
sin that escaped glaciation. Prof. Irving points out that this 
tract does not owe its immunity from glacial drift to its being 
higher than the surrounding ground ; on the contrary, it is actu- 
ally lower than the ice-ridden region in Minnesota to the west of 
it. He supposes it to have escaped because it lay between deep 
depressions leading out of Lake Superior, by which the great 
mass of ice, filling that basin, moved off to the south. If this be 
the true explanation, and it bears strong evidence of probability, 
_ | Director of the Geological Survey of Scotland. 
VOks XV.—-NO, I, I 
