PHYSICAL GEOGRAPHY OF THE PLEISTOCENE 593 
was stimulated by the increase of altitude and by the decrease of 
vegetation due to the causes already mentioned. ‘The increased rate 
of erosion led to the removal of the residual earths and alluvium which 
may well have accumulated on the surface to very considerable 
thickness, and the removal of these materials from the surface exposed 
the underlying rock to decay. 
If changes in the constitution of the atmosphere are to be regarded 
as the cause, or as even one cause of climatic change, the increase of 
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 afhrm 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 faras now known. But apart from its great climatic signifi- 
