CAUSES OF THE COLD OF THE ICE PERIOD. 287 



One result of the formation of an ice-cap over the polar regions 

 alternately in one and the other hemisphere might very well he, as 

 claimed hy Croll and admitted by Sir William Thomson, such great 

 ebhs and flows of the ocean-waters as we find recorded in the Cham- 

 plain clays, and the present depressed sea-level ; hut some more con- 

 clusive evidence of an ice-cap will be asked by cautious reasoners than 

 these alternations of level: such evidence, for example, as universal 

 glaciation over all of North America of 40 north latitude. No 

 such evidence has as yet been adduced; but, on the contrary, ob- 

 servers report an absence of ice-marks in the interior of the continent 

 northwest of the Great Lakes. This we might take to be proof that 

 the glaciers of the Ice period were limited to the highlands compar- 

 atively near the ocean, the source of evaporation, and that the inte- 

 rior was so dry then and now that no glaciers could be formed there. 

 This is, however, a subject which requires further investigation. 

 "Whatever be its cause, the uniformity and magnitude of the change 

 of sea-level from the Tertiary emergence to the Champlain submer- 

 gence, and then to the present, render it one of the most remarkable 

 phenomena recorded in geological history, and one that with careful 

 study will probably throw much light upon the great dynamical in- 

 fluences that have produced changes on the earth's surface. 



III. Either simultaneously or alternately with the extremes of 

 warmth and cold, which we find recorded in the northern, warm and 

 cold periods prevailed in the southern hemisphere. The evidences of 

 a Glacial period in South America are as conclusive as on our own 

 continent ; but it is difficult to conceive how barriers could, at that 

 time, have been thrown across the great open oceans the South At- 

 lantic and South Pacific in such a way as to confine the tropical cur- 

 rents to the central portions of these oceans. 



We are, perhaps, not justified in saying that such barriers never 

 did exist, but it will be conceded that the difficulties which oppose 

 their erection there are much greater than in the northern hemisphere ; 

 and the hypothesis which supposes their existence in the Glacial pe- 

 riod of the southern hemisphere is so entirely unsupported by facts, 

 that we are compelled to regard it as mere conjecture. 



In any discussion of the phenomena and causes of the Ice period 

 we are, up to the present time, somewhat limited and embarrassed for 

 want of a wider range of observation. The facts are not yet all in. 

 Nearly all the detailed and careful observations made on the glacial 

 phenomena of the northern hemisphere have been limited to the east- 

 ern half of North America and the western part of the European 

 Continent. Here the traces left by the glaciers are really stupendous 

 in their magnitude and extent ; and we have demonstrative evidence 

 that, during the Ice period, the glaciers and snow-fields of Greenland 

 stretched continuously clown along the Atlantic coast of North Amer- 

 ica to and below New York, and that the highlands of New England 



