372 



T. C. CHAM BERLIN 



inflow from the Atlantic. Although the Straits are shallow, the out- 

 creeping current does not appear in the upper horizons of the adjacent 

 Atlantic waters, according to Buchan's charts, but descends to depths 

 of 3,000 to 5,000 feet before it finds a horizon of density-equiHbrium. 

 It then spreads westerly in a great spatulate wedge across the North 

 Atlantic, and occupies the larger part of its area between the depths 

 of 4,000 and 5,000 feet. It is warmer and more saline than the normal 

 oceanic waters at its horizon, and lies on colder but less sahne 

 waters below. 



These and similar phenomena point to a notable closeness of the 

 balance between the density-effects of sahnity and of temperature 

 respectively. More sahne but warmer waters both overlie and 

 underlie less saline but colder waters. On the whole, however, at 

 present, the temperature effects are dominant and cold waters occupy 

 the abysmal depths of all the great oceans. 



A comparative computation of salinity-effects and of temperature- 

 effects on density, from such data as are now available, leads to a 

 similar conclusion relative to the closeness of balance between the 

 opposing agencies; but this cannot be entered upon here. 



Now, as previously remarked, the geological record gives good 

 evidence that in the majority of known periods the temperatures in 

 the polar regions were sub-tropical or warm temperate. Freezing 

 must apparently have been a trivial factor, if not quite absent, and 

 low temperature was robbed of its chief densifying effects. Evapora- 

 tion in the zones of descending air-currents in low latitudes must 

 apparently have been operative, in some degree at least, to furnish 

 the geological agencies which the record implies. Deposits of salt 

 and gypsum in not a few periods testify directly to regional aridity. 

 The most marked of these are, to be sure, referable to the periods 

 of glaciation, but many of them have no such assignable association. 



In these periods of warm polar temperatures there is reason to 

 believe that the polar temperature-effects fell below the low-latitude 

 concentration-effects, and that therefore the deep oceanic circulation 

 was actuated by the dense waters of the evaporating tracts. These 

 may then be supposed to have slowly descended and crept poleward, 

 acquiring a trivial amount of heat from the earth's interior, and 

 losing some to the vvatcrs above, but substantially maintaining 



