396 T. C. CHAMBERLIN 



the direction in which the assigned freezing action is supposed to be 

 most effective, the amount of carbon dioxide in the air often rose 

 very notably, sometimes reaching double the normal amount or 

 even more.^ The most plausible explanation of such an enrichment 

 in carbon dioxide in lands of low temperature where rapid decay is 

 out of the question and where volcanoes are unknown, is the infer- 

 ence — in itself almost a necessary one — that the cold water of the 

 Polar Sea is, as a rule, highly charged with carbon dioxide at all 

 times because of its coldness, and that the freezing of such charged 

 water in the lanes forces out the larger part of this high content of 

 carbon dioxide into the briny layer formed by such freezing, and 

 that a portion of this excess charge escapes into the air, as required 

 by the law of gaseous equilibrium. 



THE THREE MAIN LAYERS OF THE ATLANTIC COLUMN 



For the purposes of a comparison with the waters of the Polar 

 Sea, the waters of the Atlantic, though more markedly differentiated, 

 may also be grouped into three layers analogous to those of the 

 Polar Basin, an upper, a middle, and a bottom layer. This parallel- 

 ism is not only convenient for our study, but the three divisions 

 represent the great factors in the oceanic circulation of this region 

 so far as these depend on density, which is the chief cause of oceanic 

 stratification other than surficial wind action. 



I. The upper layer has considerable diversity. Its chief factors 

 are (a) poleward-moving, warm surface currents, and {h) 

 equatorward-moving, cold surface currents. These, however, do not 

 cover the whole surface. There are besides, (c) surface sheets, the 

 saHnity of which is being increased by evaporation, and this increase 

 tends to cause them to sink into the middle layer, so that they are 

 surface layers only in a transient sense. There are also, (^) waters 

 that well up from below and bear the nature of lower layers until 

 they have been "weathered" into surface layers. The last two are 

 instructive in that they represent the system of interchange between 

 the upper and lower layers and the process of transition from the one 

 to the other. The first two are the normal surface sheets. 



' Moss, "Notes on Arctic Air," Proc. Royal Dublin Soc, Vol. II; Krogh, "Abnor- 

 mal CO2 Percentage in the Air of Greenland," Meddelser on Greenland, Vol. XXVI 

 (1904), pp. 409-11. 



