THE PERMIAN PERIOD. 665 



demands than those of the coal deposits, these might apparently be met by draw- 

 ing on the stores of carbon dioxide in the air and ocean, without supposing these 

 supplies to have been very unusual. To meet the concurrent demand of lime- 

 stone formation, it may be supposed that the oceanic supply of calcium car- 

 bonate was again greatly reduced. It is possible that the volcanic and cosmic 

 contributions during the period were more than usual, but there is little clear 

 evidence of it. Considerable volcanic activity is indicated by the igneous rock 

 of the series in certain regions, but it is not extraordinary, and no very general 

 prevalence is indicated. The more probable view appears to be that the car- 

 bonates and carbon dioxide of the ocean were again reduced to a low state, and 

 that this was the condition when the deformative movements brought other 

 powerful agencies into action. 



The enrichment of the air in oxygen. — The extraction of carbon from car- 

 bon dioxide to form coal set free the associated two equivalents of oxygen, which 

 contributed to the enrichment of the atmosphere in that vital element. While 

 some oxidation was in progress during the period, the low elevation of the lands, 

 and their limited area presented conditions less favorable than usual for oxi- 

 dation. The presumption is therefore that oxygen accumulated, and that the 

 Permian period inherited from the Carboniferous an atmosphere unusually rich 

 in this vital constituent. This inference appears to be supported by the excep- 

 tional prevalence of highly oxygenated red deposits in the Permian and Triassic 

 periods, and this is perhaps their sufficient explanation, when taken in connec- 

 tion with the climatic conditions of those periods. 



The relations of equilibrium between the carbon dioxide of the air and that 

 of the ocean. — To realize how it is possible for the atmosphere to vary in its 

 content of carbon dioxide through only such moderate ranges as were compatible 

 with active air-breathing life and yet be the medium through which the ocean 

 yielded such large amounts of carbon dioxide as were demanded for the excep- 

 tional limestone and coal formations (over and above what the volcanos and 

 cosmic sources can be presumed to have supplied during these times of exceptional 

 demand), it is necessary to recognize that the carbon dioxide of the atmosphere 

 is in a certain kind of equilibrium with the carbon dioxide of the ocean. This 

 relation is peculiar and intricate, and the precise conditions and ratios of 

 equilibrium are as yet but imperfectly determined. It involves a complex series 

 of interactions between (1) anhydrous carbon dioxide (carbonic anhydride, 

 C0 2 ), (2) carbonic acid (H 2 C0 3 ), (3) calcium and other bicarbonates (Ca(HC0 3 ) 2 , 

 etc.), and (4) calcium and other monocarbonates CaC03, etc.). In the air 

 there are carbonic anhydride and carbonic acid, and in the ocean, carbon dioxide, 

 carbonic acid, calcium and other bicarbonates, and calcium and other mono- 

 carbonates. Some of these were merely mixtures whose distribution was con- 

 trolled by diffusion and mechanical stirring. Others were dissolved and held 

 by the force of solution, and others still were united chemically and held by 

 chemical forces of different strengths. For the present purposes, it may suffice 

 to note that a reduction of the carbon dioxide of the atmosphere calls forth car- 

 bon dioxide from the ocean to make up part of the loss, and bring the two into 



