Page:Popular Science Monthly Volume 59.djvu/263

Rh into oceanic depths. Low, limited lands and wide, warm seas had promoted 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 North 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 possible 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 maximum effect protected 20 per cent, of the Pleistocene land area. Continued 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) represented 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 Pleistocene 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.