OCEANIC CURRENTS. 39 



carrying each time a little sand with them. A similar phenomenon 

 has been observed on Lake Michigan ; the sands are carried steadily 

 toward the south end, where they accumulate. 



Deposits. — The invariable effect of waves, chafing back and forth 

 upon coast debris, is to wear off their angles and thus to form rounded 

 fragments and granules. Thus, pebbles, shingle, and round-grained 

 sand, though produced by all currents, are especially characteristic of 

 wave-action. Ripple-marks are also characteristic of current-action 

 in shallow water. They are, therefore, always formed on shore by the 

 action of waves and tides. By means of these characteristics of shore 

 deposit, many coast-lines of previous geological epochs have been deter- 

 mined. 



We have seen that waves usually destroy land. In many cases, 

 however, they also make land. This is the case whenever other agen- 

 cies, such as river or tidal currents, drop sediment in shallow water, 

 and therefore within reach of wave-action. We shall again speak of 

 these under the head of Land formed by Ocean Agencies. 



Oceanic Currents. 



The ocean, like the atmosphere, is iu constant motion, not only on 

 its surface, but throughout its whole mass. The general direction of 

 the currents in the two cases is also similar, but there are disturbing and 

 complicating causes peculiar to each, which interfere with the regularity 

 and simplicity of the phenomena. If the currents of the atmosphere 

 are more variable on account of the greater levity of the fluid, oceanic 

 currents have also their peculiar disturbing causes in the existence of 

 impassable barriers in the form of continents. In both atmosphere and 

 sea, currents may also be deflected by submarine banks, for mountain- 

 chains are the banks of the aerial ocean. 



Theory of Oceanic Currents. — By some distinguished physicists, 

 oceanic currents have been attributed entirely to the action of the trade- 

 winds* There can be no doubt that this is a real cause; yet it seems 

 probable, nay, almost certain, that the great and controlling cause of 

 currents of the ocean, as of the air, is difference of temperature between 

 the equatorial and polar regions, f We will, therefore, discuss the sub- 

 ject from this point of view, although the effect would be much the 

 same, whatever be our view of the theory. For the sake of clearness, 

 we will take first the simplest case, and then introduce disturbing in- 

 fluences and show their effects. 



Suppose, first, the earth covered with a universal ocean, continually 

 heated at the equator, and cooling at the poles ; the difference of den- 



* Herschel, Physical Geography, p. 13 ; and Croll, Climate and Time. 

 f Guyot, Earth and Man, p. 189. 



