WATER. 651 



age or condition ; for, whatever the temperature of the poles, 

 there must always have been a warmer tropics under the path of 

 the sun. 



The prominent characteristics of these currents bearing on their 

 mechanical effects in geological history are the following : — 



1. The rate of movement is slow. — The maximum velocity of the 

 Gulf Stream is five miles an hour, and the average less than one 

 mile and a half. 



The Gulf Stream is most rapid off Florida, where the hourly rate is three to 

 five miles; off Sandy Hook, it is one mile and a half. The rate of flow of the 

 polar current is less than one mile an hour. Kane, while shut up in the Arctic, 

 was carried south by the current, some days, about half a mile an hour. The 

 great oceanic current of the eastern South Pacific varies from three miles an 

 hour to a fraction of a mile ; and across the middle of the ocean it is barely 

 appreciable. The current in the Indian Ocean, where most rapid, has the hourly 

 rate of two miles and a quarter. 



In past geological ages the rapidity of these great oceanic cur- 

 rents must have been less than now, if there was any difference, 

 because of the less difference of temperature then between the 

 equator and the poles. 



2. The currents are generally remote from coasts, and are seldom appre- 

 ciable where the depth is less than one hundred feet, and very feeble where 

 less than one hundred fathoms. — Owing to the great depth of the 

 oceanic movement, the waters are diverted along the borders of 

 the oceans by the deep-sea slopes of the continents. In the case 

 of the Gulf Stream, these approach the coast at Cape Florida, and 

 somewhat nearly at Cape Hatteras ; but off New Jersey they are 

 eighty to one hundred miles distant; and here runs the western 

 limit of the stream. 



The polar or Labrador current, which is mostly a sub-current, 

 comes to the surface along the same slope, west of the limit of the 

 Gulf Stream, and is slightly apparent on the coast plateau, but 

 rather by its temperature than by the movement of the waters. 

 The more western position of the limit of the polar current is ex- 

 plained on page 41. The fact that it has not more rapid move- 

 ment on the great shore-plateau is evidence that it belongs to 

 the deep water. This appears further in the current's underlying 

 the Gulf Stream, and its banding the stream with colder and 

 warmer waters, as shown by the Coast Survey under Professor 

 Bache. The observations of the survey have proved that there 

 are mountain-ridges apparently parallel with the Appalachians 

 along the course of the stream in its more southern part, and that 

 above these ridges the surface-waters are cooler, owing to the lifting 



