ciiAP. v.] GENERAL CONCLUSIONS. 313 



Mr. Buchanan drew the conchision, in expLanation of the 

 small amount of oxygen at depths of 300 fatlioms and upward, 

 ''that animal life must be particularly abundant and active at 

 this depth, or at least more abundant than at greater depths." 

 In other words, that a permanent condition, probably of all con- 

 ditions the most unfavorable to animal life, is produced and 

 maintained by its excess. 



This is entirely contrary to experience. I think, however, 

 that the observation, which is in itself of the highest interest, 

 goes far to support the opposite opinion, at which I had previ- 

 ously arrived from other considerations, that in deep water a 

 wide intermediate zone between the surface and the layer im- 

 mediately above the bottom is nearly destitute of animal life — 

 at all events, in its higher manifestations. 



If the view which I have adopted of the cause and course of 

 the circulation of the water in the Atlantic and Pacific oceans 

 be correct, it seems to afford a ready explanation of the j^eculiar 

 distribution of oxygen. Free oxygen is doubtless in all cases 

 derived l)y the water of the sea from the atmosphere, and it is 

 consequently absorbed through the surface, where the water is 

 constantly agitated in contact with the air, and the surface-wa- 

 ter contains most. 



In the Antarctic regions, the surface-water sinks raj)idly to the 

 bottom, and moves northward as the cold southern indraught. 

 The bottom-water has thus, next to the surface-water, had the 

 latest opportunity of becoming impregnated with air, and a con- 

 siderable portion of that air it retains. If the deep circulation 

 in the Atlantic and the Pacific be chiefly maintained, as I have 

 been led to believe, by evaporation of the surface-water and a 

 slow inclraught of Antarctic water beneath to supply its place, a 

 central belt, or, at all events, a belt at too great a depth to be 

 affected by surface influences, must be the oldest water in the 

 vertical section, and must consequently have been longest sub- 

 jected to the removal of oxygen by the scantv fauna which may 

 II.— 21 



