THE OCEAN. 659 



southern part, off the Carolinas, and that, above these ridges, the sur- 

 face-waters are cooler, owing to the lifting upward of the polar current 

 by the submarine elevations. The fact, that the cold waters produce 

 a temperature of 35° F., at a depth of six hundred fathoms, off 

 Havana (as stated by Bache), is proof of the great magnitude of the 

 polar current. 



Where the current flows close along a coast or submarine bank, or 

 by an oceanic island, it may produce some eroding effects. 



3. As the position of the main flow of the currents is determined 

 partly by the trend of the continents, their courses may have been differ- 

 ent in former time from what they are now, provided the continents, or 

 large portions of them, were sufficiently submerged. — Small subsidences 

 would not suffice to produce a diversion from their present courses, for 

 the reason just given. Even the barrier of Darien might be removed, 

 by submergence to a depth of five hundred feet, and probably one 

 thousand, without giving passage to much, if any, of the Gulf Stream. 

 If, however, the straits were so deeply sunk that the Gulf Stream 

 passed freely into the Pacific (the West India islands being also in the 

 depths of the ocean, as would be necessary for the result), a great 

 change would thereby be produced in the temperature of both the 

 Atlantic and the Pacific, — a loss of heat to the former and a gain to 

 the latter. (See Physiographic Chart.) But no facts yet observed 

 prove this supposition to have been a realized fact since the opening 

 of the Silurian age. A shallow-water connection across the isthmus 

 between the two oceans probably existed as late as the Cretaceous, as 

 has been inferred from the parallel series of representative species now 

 existing on the two sides. 



Besides the general system of currents, which has been considered, there are currents 

 between the ocean and some confined seas opening into it, which are due to the evap- 

 oration going on over the surface of those seas. The consequent diminution of water 

 causes a flow at surface from the ocean, to supply the loss. This happens at the Straits 

 of Gibraltar, opening into the Mediterranean. At bottom, there is a flow outward, of the 

 denser water. In many seas of this kind, the accessions from rivers more than supply 

 the amount removed by evaporation ; and these produce an out-current at the entrance. 

 The Black Sea, by losing much of- its salt, is rendered less dense than the iEgeau, to 

 the south, and hence there is an under-current into it at the Dardanelles. 



2. Tidal Waves and Currents. 



1. Rise and Fall of Tides. — The simplest of tidal actions is the 

 periodical rising of the waters on a coast. The in-flow acts like a 

 dam, in setting back the waters of springs and rivers. It floods large 

 areas on flat coasts, which are thereby made salt-marshes. 



The height of the tide is less in mid-ocean than along the continents, 

 and is greatly augmented where two coast-lines converge, as on enter- 



