206 



CORALS AND CORAL LSL.ANDS. 



marine currents in keeping the bottom dear of growing coral. 

 These are the principal means by which fresh-water streams 

 contribute toward determining the existence of harbours ; 

 for little is due to their freshening the salt waters of the 

 sea. 



The small influence of the last-mentioned cause — the one 

 most commonly appealed to — will be obvious, when we 

 consider the size of the streams of the Pacific islands, and the 

 fact that fresh water is lighter than salt, and therefore, instead 

 of sinking, flows on over its surface. The deepest rivers 

 are seldom over six feet, even at their mouths ; and three or 



PART OF THE NOK'IH SHORE OF lAHITl. 



four feet is a more usual depth. They will have little effect, 

 therefore, on the sea water beneath this depth, for they cannot 

 sink below it ; and corals may consequently grow even in 

 front of a river's mouth. Moreover the river water becomes 

 mingled with the salt, and, in most cases, a short distance out, 

 would not be unfit for some species of coral zoophytes. 



Fresh-water streams, acting in all the different modes pointed 

 out, are of little importance in harbour-making about the 

 islands of the Pacific. The harbours, with scarcely an excep- 

 tion, would have existed without them. They tend, however, 



