242 THREE CRUISES OF THE " BLAKE." 



present epoch the most marked physical feature of the surface 

 of the glohe is its subdivision into a land and an oceanic hemi- 

 sphere. Thomson, like him, looks upon the oceans as contin- 

 uous, and has happily styled the Atlantic, the Pacific, and the 

 Indian oceans as great gulfs of the Southern Ocean. 



The striking hydrographic character of the North Atlantic is 

 its comparative isolation from the Arctic Ocean ; the South At- 

 lantic, on the contrary, is fully open to the cu-culation of cold 

 water coming from the Antarctic Ocean. (See Fig. 162.) The 

 South Atlantic is shut off from its northern area by the ridge ex- 

 tending from St. Paul's Rocks to Ascension, at a depth of about 

 2,000 fathoms. The Challenger Ridge runs nearly north and 

 south, leaving a free communication between the Antarctic 

 Ocean and the eastern and western basins of the South Atlantic. 

 The North Atlantic is subdivided into an eastern and western 

 basin at a depth of about 1,500 fathoms by the Dolphin Rise, 

 which follows in a general way the course of the S-shaped At- 

 lantic basin. Ridges separating the Atlantic from the Arctic 

 Ocean extend across Denmark Straits, probably at a shallow 

 depth. From Greenland to Iceland the depth has an average of 

 500 fathoms ; from Iceland to the Fseroes, an average of about 

 300 fathoms ; and from there to the Orkneys, of not more than 

 220 fathoms. From the configuration of the bottom (see Fig. 

 61), it is evident that a larger amount of cold water must reach 

 the tropics from the antarctic than from the arctic regions,^ 

 which are shut off from the Atlantic by submarine ridges. Over 

 these, and through the channels of Baffin's Bay, but a limited 



^ The temperature line run cliagoually colder belt adjoining the equatorial re- 



across the Atlantic from Madeira to gion. In other words, the cold water 



Tristan da Cuuha by the " Challenger " may be said to rise towards the surface 



brings out the remarkably shallow stra- near the equator ; and from the tempera- 



tum of warm water of that part of the ture of the two sides of the North Atlan- 



equatorial regions which corresponds to tic it is also evident that the supply of 



the regions of the tradewinds both north cold water flowing from the Antarctic 



and south of the equator. The tempera- into the Atlantic is greater than that com- 



tures of the belts of water between 200 iug ivom the arctic regions. This vertical 



and 500 fathoms north and south of the circulation, characteristic of the equato- 



line plainly show that the colder water rial belt, is insignificant, however, when 



found south of the equator cannot come compared with the great horizontal oce- 



from the warmer northern belt of the anic currents, 

 same depth, but must come from the 



