STRUCTURE OF CORAL REEFS. 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 
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 débris 
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- 
