658 DYNAMICAL GEOLOGY. 



1. General System of Currents. 



The system of oceanic currents is briefly explained on pages 38- 

 42. It is part of the organic structure of the globe, irrespective of 

 its 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 cur- 

 rent 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 currents 

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

 of the less difference of temperature between the equator and the 

 poles, and hence feebler trade-winds. 



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

 appreciable 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 and Cape Cod ; but, off New Jer- 

 sey, 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 shore-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 explained on page 

 39. The fact that it has not more rapid movement, 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 sur- 

 vey have proved that there are mountain-ridges apparently parallel 

 with the Appalachians, along the course of the stream, in its more 



