CLIMATE AND CARBONIC ACID. 253 



into oceanic depths. Low, limited lands and wide, warm seas had pro- 

 moted the flow of carbon dioxide from the waters to the air. Lands 

 elevated and expanded and seas shrunk within their basins reversed the 

 course, and the earth took from the air to give to the waters. 



The rate of depletion is capable of reasonable calculation. If the 

 amount of carbon dioxide taken from the atmosphere exceeded by 10 

 per cent, that supplied to it from all possible sources, 50,000 years 

 would suffice to reduce the content from .18 per cent, to .03 per cent, 

 by weight. And this change would bring on glaciation. There are few 

 students of the earth's history who would be willing to admit that the 

 associated effects of topographic development could have occurred in 

 less, if, indeed, in so short a time, and the causes assigned are thus seen 

 to be fully equal to the task imposed. 



The climates of the Glacial period were marked by rhythm recorded 

 in advance and retreat, and re-advance and withdrawal, of the ice front 

 several times repeated. The major changes were as great as that which 

 has intervened between the severest glaciation and the present, and 

 occurred early in the series. The later oscillations declined in both 

 Europe and jSTorth America. Such rhythmic rebound from one phase 

 to another and back again is characteristic of phenomena which, 

 though they swing to extremes, themselves set up the action that 

 reverses the movement. The ice sheet itself set the bounds of its pos- 

 sible spread. 



Assuming glaciation to be inaugurated and the cold to be intensified 

 by consequent accelerating influences, which need not be detailed here, 

 the depleting process of weathering must be checked by the mantling 

 ice and refrigeration. It is estimated that frost and ice at their maxi- 

 mum eifect protected 20 per cent, of the Pleistocene land area. Con- 

 tinued depletion depended on the balance between contribution and 

 abstraction, and it is suggested that 20 per cent, (or whatever may 

 have been the proportion of land area sealed against carbonation) rep- 

 resented the initial preponderance of draft over supply. Whenever the 

 effects of glaciation reduced the consumption of carbon dioxide below 

 the inflow from all sources, the glacial epoch would end and the reaction 

 would begin. Once initiated, it would be accelerated by diffusion and 

 dissociation in the richly stored seas, and by renewed development of 

 life in the warmer waters. The mildness might increase till the great 

 glaciers had vanished, but it could have come to stay only in case the 

 height and area of land had adequately diminished. 



Lands remained extensive and elevations great during the Pleisto- 

 cene period. They were even wider and higher than they are now. 

 As an early ice mantle shrunk it bared rock masses and glacial deposits, 

 which were to a great extent favorably conditioned for chemical attack. 

 The renewed consumption of carbon dioxide in time overbalanced the 

 supply, and glaciation went on again. 



