THE PLEISTOCENE OR GLACIAL PERIOD. 443 



to reduce the period, and the uncertainties of this estimate are not 

 unlike those relative to the length of a glacial advance or retreat, but 

 the period thus estimated is of the same general order of magnitude 

 as that of the glacial stages, and nothing beyond such a similarity in 

 order of magnitude is to be expected. During this process of higher 

 carbonation of the ocean, the advance of the ice was reducing the 

 area of the land exposed to carbonation, and was thus reducing the 

 carbonation of the rocks. This checking of the carbonation on the 

 land cooperated with the reversal of sea-action in the inauguration of 

 an ice retreat. 



As warmth increased there should have been, normally, an increase 

 of lime-secreting plants and animals, and these would have secreted 

 more lime individually, as a rule, setting free more of the second equiv- 

 alent of carbon dioxide of the calcium carbonate. The moisture in 

 the air should have increased with the increase of warmth and the 

 melting of the ice-fields. This new combination gained in force as 

 the ice was removed. It is assumed that the cooperative force of this 

 combination, once in dominance, maintained its superiority over the 

 opposing agencies until the ice-sheets were largely or wholly removed, 

 and the freezing that had inaugurated the oceanic super-carbonation 

 ceased to be effective. 



When the full land-surface was again exposed to carbonation, and 

 the air had been re-enriched in carbon dioxide, and the oceanic cir- 

 culation had carried the most highly carbonated portions of its waters 

 to the surface in low latitudes, and had begun to bring up the rela- 

 tively low carbonated portion that had descended in high latitudes 

 after the carbon dioxide had become depleted to its lowest state, the 

 conditions were ripe for a new process of depletion and glaciation 

 under conditions closely similar to the previous one. The process 

 could thus be repeated until the general conditions that brought on 

 the glaciation ceased to be effective, and the conditions for re-inau- 

 gurating a movement toward a mild uniform climate were restored. 

 It is not presumed, however, that the oceanic circulation was reversed 

 in the interglacial stages, but that the super-carbonation in the high 

 latitudes was reduced to an ineffective measure, or stopped entirely. 

 In a climate that permitted pawpaws and osage oranges to nourish in 

 eastern North America above latitude 43°, and induced lions, leopards, 

 hippopotamuses, etc., to invade the middle latitudes of Europe, an 



