PHYSICAL GEOGRAPHY OF THE PLEISTOCENE 269 



erosion at the close of the Tertiary would have led to an increased 

 consumption of carbon dioxide, and so may have been responsible 

 for the initial step in the series of changes which brought on the glacial 

 climate. Though it is, perhaps, too early to affirm that the increased 

 altitude of the land at this time was the basal cause which led to the 

 cold climate which followed, this is a hypothesis toward which students 

 of glacial geology are looking with much hope. 



If the increased height of the land led to increased erosion, and so 

 to increased consumption of carbon dioxide, the reduction of the 

 amount of this gas in the atmosphere would have lowered the tempera- 

 ture everywhere. The resulting decrease in the temperature of the 

 sea would have led to an increased solution of carbon dioxide from 

 the atmosphere, thus depleting the atmospheric supply still further, 

 and this, in turn, reacted upon the temperature and became a cause 

 of its further reduction. This cause, therefore, once in operation, 

 must have continued with increased effectiveness until the decay of 

 rock was checked by decrease of altitude or temperature, or by the 

 accumulation of ice-sheets which protected the rock beneath from 

 ready carbonation. 



III. THE DIRECT IMPORTANCE OF THE ICE-SHEETS THEMSELVES 



Irrespective of the cause of the glacial climate, the covering of six 

 million or more square miles of land in the northern hemisphere with 

 ice hundreds and thousands of feet in thickness was in itself an extra- 

 ordinary event which might well serve as an important landmark in 

 geologic history. The ice-sheets, and especially the remarkable 

 successions of ice-sheets, might appropriately be emphasized as proof 

 of one of the most remarkable climatic incidents in the history of the 

 earth, so far as now known. But apart from its great climatic signifi- 

 cance, each ice-sheet meant the relatively rapid superposition upon the 

 northern continents, over the great areas indicated, of a new layer of 

 rock, the ice, which for tracts of millions of square miles must have 

 had a thickness exceeding a thousand feet, and perhaps a thickness of 

 several thousand feet. The aggregate volume of this new rock, 

 superposed on the northern parts of the northern continents, was such 

 that it could only have been measured in terms of millions of cubic 

 miles. The withdrawal of its substance from the sea effected a cor- 



