652 DYNAMICAL GEOLOGY. 



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 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 different in former 

 time from what they are now, provided the continents, or large portions of 

 them, loere 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 sub- 

 mergence to a depth of five hundred feet, and probably one thou- 

 sand, 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 both 

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

 gain to the latter (see Physiographic Chart). But no facts yet ob- 

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

 opening of the Silurian age. 



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 evaporation going on over the surface of those seas. The conse- 

 quent diminution of water causes a flow from the ocean to supply the loss. 

 This happens at the Straits of Gibraltar opening into the Mediterranean. 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. 



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 conti- 

 nents, and is greatly augmented where the two coast-lines con- 

 verge, as on entering a bay, and especially where there is free 

 entrance to a channel from two directions. In the middle At- 

 lantic, at St. Helena, it is two or three feet ; at the Azores, three 

 feet ; on the Atlantic coast of the United States, from five to twelve 

 feet ; but in the Bay of Fundy, fifty to seventy feet. In the cen- 



