42 CORAL FORMATIONS. 
it had the consistence of mud, and was seldom observed to be 
covered with coarse material. There were some small patches of 
coral over it, and here and there a growing mass of Porites. The 
fresh waters of the shores do not flow over these wide reefs, as there is 
no proper inner channel, and there is consequently no shore detritus 
mingled with the reef debris. At ‘Tahiti, the sounding lead usually 
brought up sand, shells, and fragments of coral. At 'Tongatabu, the 
bottom, where the Peacock anchored, was a grayish blue mud, ap- 
pearing as plastic as common clay; it consisted solely of comminuted 
coral and shells, with colouring matter probably from vegetable 
decomposition. ‘To the west of the larger Feejee islands, soundings 
commonly indicated a bottom of basaltic mud, and this material was 
frequently brought up with our dredges. On the north side of 
Vanua Lebu, a stream has so filled with its detritus the wide chan- 
nel into which it empties, that for a mile our ship dragged its keel 
in the mud, although elsewhere the water had been from twelve to 
twenty fathoms deep; and at least half a dozen square miles of land 
had been added to the shores from this source. Though due 
principally to shore material, the reefs have probably added some- 
what to these accumulations; yet little coral sand can be detected 
in the mud by the eye, and the proportion is certainly very 
small. In many places where we anchored, having the reef not 
more than five hundred yards from the ship, we might have judged, 
from the character of the bottom, that there were no corals nor 
shells within 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 water 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 debris from some regions to 
others distant ; and again they bear along only the shore detritus, and 
distribute it. 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 basaltic island, 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 Bacon’s Isles, as a mere point of rock in a wide sea 
