150 C0RAL8 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 mad ; 

 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 debris), 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 debris. 



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 

 from this source. Though due principally to shore material, 



