242, THREE CRUISES OF THE “ BLAKE." 
present epoch the most marked physical feature of the surface 
of the globe 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 circulation 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 Feries, 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 
1 The temperature line run diagonally 
across the Atlantic from Madeira to 
Tristan da Cunha by the “ Challenger ” 
brings out the remarkably shallow stra- 
tum of warm water of that part of the 
equatorial regions which corresponds to 
the regions of the tradewinds both north 
and south of the equator. The tempera- 
tures of the belts of water between 200 
and 500 fathoms north and south of the 
line plainly show that the colder water 
found south of the equator cannot come 
from the warmer northern belt of the 
same depth, but must come from the 
colder belt adjoining the equatorial re- 
gion. In other words, the cold water 
may be said to rise towards the surface 
near the equator ; and from the tempera- 
ture of the two sides of the North Atlan- 
tic it is also evident that the supply of 
cold water flowing from the Antarctic 
into the Atlantic is greater than that com- 
ing from the arctic regions. This vertical 
circulation, characteristic of the equato- 
rial belt, is insignificant, however, when 
compared with the great horizontal oce- 
anie currents. 
