668 GEOLOGY. 



of equilibrium in the polar regions. In such a case, in passing from the one 

 to the other, the surface waters must be absorbing carbon dioxide, as long ago 

 recognized by Dittmar and others. Relative to the average state of equilibrium 

 for the whole contact surface, whatever that may be, the equatorial portion 

 must be more highly charged, other things being equal, and the polar portion 

 less highly charged. If the average state at the surface is near equilibrium, 

 the equatorial portion is probably emitting carbon dioxide and the polar por- 

 tion absorbing it. This is probably the actual condition now and perhaps has 

 been generally in those geologic ages in which the climatic states were similar to 

 those of the present. It may be accepted as a general proposition that, in 

 the course of the abysmal circulation, essentially all oceanic waters pass through 

 the absorbing stage in the polar regions and the emission stage in the equatorial 

 regions, and that the adjustments of equilibrium for the great mass of the ocean 

 are dependent on this slow circulation, whose period may equal a glacial epoch. 

 In estimating it, the more active wind-actuated circulation of the surface must 

 apparently be set aside, in the main, as having little to do with the abysmal 

 circulation. It has recently been shown in the investigation of small lakes, 

 whose depths rarely reach 200 feet, that the wind-driven circulation has little 

 effect on the basal waters, but develops a surprisingly shallow superficial zone 

 of its own. The waters below this are sometimes so far unaerated, or so charged 

 with noxious gases, as to be offensive and inhospitable to life. 1 The tempera- 

 tures of the deeper waters of lakes, however, show that there is a seasonal descent 

 of cold waters analogous to the abysmal circulation of the ocean, but essentially 

 independent of the eolian circulation. (For the hypothetical reversal of circulation, 

 see pp. 432-446, Vol. III.) The discussion here will proceed on the supposition 

 that the present deep-sea circulation was persistent. 



The influence of secular temperature changes. — If the polar temperatures in 

 the mild stage previous to the Permian period averaged about 20° C, as the 

 life seems to imply, the abysmal waters would become carbonated on the basis 

 of polar absorption at 20° C. If, other conditions remaining the same, the polar 

 temperature at the time of the Permian glaciation was reduced somewhat below 

 the present temperature, the basis of equilibrium must have shifted to absorption 

 at 0° C. to — 4° C, or to an absorption ratio about double its previous one. In 

 making the adjustment to the new basis of equilibrium, corresponding drafts 

 on the atmospheric carbon dioxide are to be inferred. It follows from these 

 considerations that at a time when the polar temperatures were being lowered, 

 and the basis of equilibrium was being changed adversely to the atmospheric 

 content of carbon dioxide, the latter was ill conditioned for meeting exceptional 

 demands made upon it from other sources, 



When this critical state, arising from the very conditions of equilibrium itself, 

 is taken in connection with the depleted state of the oceanic carbon dioxide, 

 arisino- from previous exceptional extraction to form coal and limestone, it will 

 be apparent that the Permian deformation was liable to produce unusual effects. 



1 E. A. Birge, Wis. Geol. and Nat. Hist. Surv. 



