150 CORALS AND CORAL ISLANDS. 
Yet the bottoms of these channels are not always made up 
of calcareous or coral sands and fragments; for the volcanic 
or basaltic lands they adjoin are a source of ordinary mud; 
and the river courses of the land and the tidal currents of the 
sea will often determine the nature of the bottom, or may | 
cause in it alternate variations. 
At Upolu the white coral sands of the reefs (or in more 
general terms the reef débris), forms the bottom. In some 
places this coral material had the consistence of mud, and it 
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 débris. 
At Tahiti, the sounding lead, where dropped in the channels, 
usually brought up sand, shells, and fragments of coral. At 
Tongatabu, the bottom where the Peacock anchored was a 
grayish blue calcareous mud, appearing as plastic as common 
clay ; it consisted solely of comminuted corals and shells, with 
coloring matter probably from vegetable and animal decom- 
position. 
But to the west of the larger Feejee islands, in the channels 
near the land, soundings commonly indicated a bottom of mud 
made from the material of the rocks of the mountains, and the 
same was frequently brought up with our dredges. On the 
north side of Vanua Lebu, a stream had so filled with its de- 
tritus the wide channel into which it empties, that for a mile 
the depth is but two to three fathoms, although elsewhere the 
depth is mostly from twelve to twenty fathoms; and at least 
half a dozen square miles of land had been added to the shores 
trom this source. Though due principally to shore material. 
