GLACIAL DRIFT. 329 
heights of land and sea during the glacial and Champlain periods. They 
are the result of much reflection upon his part, and are worthy of careful 
consideration. 
CHANGES IN THE RELATIVE HeIcHTs or LAND AND SEA DURING THE 
GLACIAL AND CHAMPLAIN PERIODS. 
By Warren UrHam. 
Evidence has been found of the accumulation of immense ice-sheets, 
probably in the same epoch, over the north part of America, Europe, and 
Asia, also in New Zealand and the south part of South America. This 
‘points to some cosmical cause for the glacial period, like that assigned by 
Croll, rather than to the causes appealed to by Lyell and Dana, namely, 
elevations of the earth’s crust. It would be a very improbable coinci- 
dence that such extensive regions surrounding both poles should be thus 
elevated in the same period; but these would obviously be the parts of 
the globe to be covered by ice if its origin was due to eccentricity of the 
earth’s orbit. There seem to be reasons (pp. 7-9), however, to discredit 
the conclusion of Croll, that the ice-sheets were alternately wholly melted 
away in each hemisphere once in every 21,000 years; instead of which 
it seems probable that the ice-mantle existed upon both hemispheres at 
the same time. 
The effect of this extraordinary accumulation of ice about the poles 
would be to take away a large amount of water from the ocean, and, fur- 
thermore, to draw the sea, by gravitation, away from the equator, leaving 
the sea-level lower than now within the tropics, but at the same time 
causing it to rise even higher than now near the lower limit of the ice- 
sheets, and much higher than now near the poles. Marine shells in the 
modified drift show that the sea thus rose 150 to 200 feet above its pres- 
ent height in the latitude of New York city; 500 feet in the valley of the 
St. Lawrence; and 1,000 to 2,000 feet in arctic regions. Everywhere in 
high latitudes, both in the northern and southern hemispheres, we have 
proof of such a submergence of the land when the drift was accumulated, 
increasing in amount the nearer we go to the poles. On the other hand, 
the coral islands of the tropics are witnesses of a depression of the sea, 
amounting to 3,000 feet or perhaps much more at the equator, while dif- 
ferent proof shows that at the mouths of the Mississippi, Ganges, and 
