STRUCTURE OF CORAL BEEFS. 151 



the reefs have probably added somewhat to these accumula- 

 tions; yet .little coral sand could be detected in the mud by 

 the eye, and the proportion is certainly very small. In many 

 places where the ships of the Wilkes Exploring Expedition an- 

 chored, having the reef not more than five hundred yards from 



» 



the ship, the material of the bottom was wholly mud from 

 the land, as much so as if there were no corals or shells with- 

 in many miles. 



When the materials from both sources, the shore and the 

 reef, are mingled, the proportion ' will necessarily depend on 

 the proximity to the mouths of streams, the breadth of the 

 inner waters or channels, and the direction and force of the 

 currents. These tidal currents often have great strength, and 



O o 7 



are much modified and increased in force at certain places, or 

 diminished in others, by the position of the reef with reference 

 to the land. Sweeping on, they carry off" the coral debris 

 from some regions to others distant ; and again they bear along 

 and distribute only the shore detritus. It is thus seen that 

 the same region may differ widely in its adjacent parts, and 

 seemingly afford evidence in one place that there is no coral 

 near, and in another no high land, although either is within a 

 few rods, or even close alongside. 



The extent of the land in proportion to the reef will have 

 an obvious effect upon the character of the channel or lagoon 

 depositions. When the island stands, like one of Bacon's isles 

 in the Feejees, as a mere point of rock in a wide sea en- 

 closed by a distant barrier, the streams of the land are small 

 and their detritus quite limited in amount. In such a case, 

 the reef, and the growing patches scattered over the lagoon, are 

 the sources of nearly all the material that is accumulated upon 

 the bottom. 



The bottom between the inner reefs within the great Aus- 



