242 



CORALS AND CORAL ISLANDS. 



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

 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 harbor-making about 

 the islands of the Pacific. The harbors, with scarcely an ex- 

 ception would have existed without them. They tend, how- 

 ever, by the detritus which they deposit, to keep the bottom 



PART OF NORTH SHORE OP TAHITI. 



more free from growing patches of coral, and consequently 

 produce better anchorage ground ; moreover, within the har- 

 bors they usually keep channels open through, or over, the shore 

 reef sufficiently deep and wide for a boat to reach the land, 

 and sometimes preserve a clean sand-beach throughout. That 

 this is their principal effect will appear from a few facts. 

 The map of the reef of North Tahiti, between Papieti on 



