HVPOTAHESIS OF CAUSE OF GLACTAL PERIODS 673 
of withdrawal, combined with associated agencies, were not 
sufficient to arrest the progress of glaciation, the glaciation 
would have continued to extend itself until the point of balance 
was reached. It is, perhaps, not too much to assume that the 
extension of glaciation and its concurrent agencies mark in 
themselves the measure of preponderance of the depleting 
agencies at the stage when glaciation began. That is to say, 
after the depleting agencies had brought the air’s carbonic 
content down to the point at which the glacial centers were 
inaugurated, these agencies were still preponderant over the 
repleting agencies to some such extent as 20 per cent., more or 
less. 
While it does not seem necessary to our general purposes to 
consider the associated agencies of arrest, if these views be cor- 
rect, they possess an interest of their own, and may be men- 
tioned briefly. 
With little doubt the lowering of the temperature lessened 
the rate of decomposition of the silicates, though frost action 
aided in disaggregating them mechanically and in thus increas- 
ing the atmospheric contact. On the other hand, while the 
authorities are not altogether agreed, the weight of the latest 
opinion supports the view that the limestones would be dissolved 
by cold water saturated with carbon dioxide faster than by warm 
water, other things being equal. The cold waters would quite 
certainly contain more absorbed carbon dioxide than warm 
ones. Probably other things were not equal, for the vegetable 
action and the organic acids probably lent less and less aid as 
the temperature fell. It is not clear what the balance of these 
influences combined would be. Whichever way it leaned, it 
does not seem to have been of decisive moment. 
As previously remarked, the progressive removal of carbon 
‘dioxide from the air reduced the amount of its action, but not 
proportionately. While this lessened the depleting action it 
does not seem to have reached decisive moment, at least not 
until a late stage in the process of depletion. 
Meanwhile, in the ocean, conditions favoring reaction were 
