302 THREE CRUISES OF THE " BLAKE." 



between that of 38° and one near the freezing point. Of course, 

 the depth at which the belt reaches the surface will change ac- 

 cording to the latitude of the intermediate zones of cold and of 

 the coldest water. The belt of variable heat is deepest in the 

 tropics, while the belt of maximum cold reaches the surface in 

 hia'h latitudes. 



In these belts of temperature, we can most satisfactorily trace 

 the so-called faunal districts into which the shores of our conti- 

 nents have been subdivided. The recognized geographical pro- 

 vinces, cbcumscribed, as first pointed out by Dana, by the lowest 

 degree of cold to which they are subject, comprise, as a general 

 rule, animals living along the shores stretching north and south 

 within certain limits. The bathymetrical range of these lit- 

 toral species is small, and with few exceptions they do not live 

 within the limits of either the continental or the abyssal areas, 

 where flourish the faunae of the other two belts. The tempera- 

 ture of the continental belt is sufficiently low to crop out in 

 high latitudes, and hence the large number of so-called arctic 

 species which have been found to be quite common in moderate 

 depths upon this so-called continental area ; while the strictly 

 deep-sea or abyssal forms rarely occur within the range of the 

 continental, and still more rarely within that of the littoral belt. 



In making a comparison between the atmospheric and watery 

 envelopes of the earth, it is interesting to compare the high 

 velocity of the atmospheric currents and their cataclysmic dis- 

 turbances with the slow movements of the strongest oceanic 

 currents, — the direction of the atmospheric currents being more 

 nearly what those of the ocean would be were it not for the 

 disturbing effects of continental masses ; to the action of these 

 currents we undoubtedly owe the general drift of the surface 

 flow of the oceanic basins. This, in connection with the ther- 

 mal differences and variations in specific gTavity, would produce 

 surface and under currents, modified in their turn by the rotation 

 of the earth. 



The general conditions of temperature below five hundred 

 fathoms are very simple. From there an immense body of 

 cold water with a temperature of less than 40° (as low as 33.36°, 

 or near the freezing point), stretches to the bottom of the 



