NORTH AMERICA IN THE ICE PERIOD. 5 



is unmistakable, and no one who has not learned the language in which 

 it is written is warranted in taking part in the discussion ; but he who 

 has done so will find graven on the rocks of the Alps, the hills of New 

 England, the basins of the Great Lakes, and the mountains of Colorado 

 and Oregon, an inscription which is everywhere the same, which can have 

 but one meaning, and bears a signature that can not be counterfeited. 



While it is hopeless to expect that all men will agree upon this — 

 or any other — subject, I think I am justified in saying that the facts 

 which have been stated, and others of like import, constitute an indis- 

 putable record, not necessarily of the former existence of a great ice- 

 cap over all the northern regions, but of the simultaneous prevalence 

 of sheets of land-ice, i. e., glaciers, over great areas of our continent ; 

 and that these glaciers, forever in motion, holding imbedded in their 

 substance sand, gravel, and bowlders, pressed against the underlying 

 rock by their enormous weight (probably averaging fifty thousand 

 pounds to the square foot),* became powerful agents of erosion ; gen- 

 eral and uniform when they were broad, narrow and special when they 

 were local. This is the reading of the facts now given by those who 

 are best qualified to judge of the import of the phenomena in the Old 

 World and in the New. Already the belief in an ice period and 

 ancient glaciers is general — hereafter, w T ith more complete knowledge 

 of the subject, it must become universal. 



Accepting the facts cited above as demonstrating the truth of the 

 glacial hypothesis, and as proving beyond cavil the reality of an ice 

 period, we now pass to consider the proximate and remote causes of 

 the distinctive phenomena of this remarkable chapter in geological 

 history. 



With characteristic conservatism Lyell endeavored to account for 

 the prevalence of glaciers over the northern hemisphere by supposing 

 them to be due to a peculiar arrangement of land and sea ; broad 

 and elevated areas of land in the Arctic regions, low and narrow land 

 surfaces in the tropics. I have elsewhere f discussed this question at 

 some length, and have shown that this theory is untenable, because : 

 First, during the Tertiary age the land was high at the north, no ma- 

 rine Tertiary deposits being found there ; Asia, Europe, and Ameri- 

 ca were then connected by land, and the tropical currents were 

 excluded from the Arctic Ocean, J but m that age a warm climate 



* Fifty-four thousand eight hundred and ten pounds for one thousand feet in thick- 

 ness ; in some cases (around Mount Washington), probably two hundred and fifty thousand 

 pounds. 



f " Popular Science Monthly," July, 1876. 



% At least through the channels of the North Pacific or North Atlantic. It has been 

 suggested that in the Tertiary ages a communication existed between the Mediterranean 

 and the Arctic Ocean by way of the Caspian Sea, Sea of Azov, etc. ; but if there was an 

 open channel across Western Asia at that time — which has not been proved — it could 

 hardly have been broad and deep enough to permit a flow through it both ways (for no 

 other channel is known) of sufficient volume to modify the climate of the Arctic regions. 



