442 GEOLOGY 



The depths of the ocean are now filled with water but little above 

 the freezing-point, which implies a deep-seated movement from the 

 polar regions. This goes to show that diffusion, mechanical mixture, 

 friction, agitation transmitted from the surface, tidal and earthquake 

 motions, and the internal heat of the earth, all combined, do not more 

 than slightly modify the dominance of this circulation as a means of 

 determining the temperature of the deep sea, and hence there is still 

 less reason to question its dominance in determining the saline and 

 gaseous content of the deep-sea waters, for only the first two of the 

 agencies tend to diffuse these constituents. The form of the super- 

 carbonation is indeed changed by the solution of minute calcareous 

 shells that fall from the ocean surface, and are dissolved before they 

 reach the greatest depths, as shown by the Challenger investigations; 

 but the carbon dioxide so used becomes free again when the calcium 

 carbonate is again secreted by plants or animals. The point of moment 

 here is that the process is essentially one of circulation, and is not 

 essentially modified by diffusive processes, and hence that the time- 

 period is closely measured by the great cycle which carries the whole 

 body of the ocean through its concentrating action. The relatively 

 rapid surface circulation of the ocean has little to do with this. 



According to the observations of Peary 1 and Nansen 2 the first 

 season's freezing at the points of their observations, which may be 

 called mid-arctic, reaches depths of 4 to 8 feet. That of subsequent 

 seasons, when the old ice remains, is appreciably less. In the center 

 of the frozen seas, the old ice forms a persistent covering. If a layer 

 of new ice as much as 5 feet in thickness were formed annually over 

 an area of 9,000,000 square miles — about the area of the Arctic and 

 Antarctic Oceans combined, according to Murray — a mass of water 

 equal to that of the whole ocean would pass through the freezing 

 process in about 33,000 years; if the annual layer were 3 feet thick, 

 in about 55,000 years; if 2 feet, in about 83,000 years. This implies 

 a movement equal to the amount of freezing only, and a correspond- 

 ingly high concentration of salt and gas. A greater movement and 

 a less concentration are much more probable, and hence a shorter 

 period for the super-carbonating epoch. There is a considerable list 

 of modifying conditions, the most of which would apparently tend 



1 Personal information. 



2 Scottish Geog. Mag., Vol. XIII, 1897, p. 240. 



