I20 CORALS AND CORAL LS LANDS. 



At Upolu the white coral sands of the reefs (or in more 

 general terms the reef debris) form 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 

 greyish blue calcareous mud, appearing as plastic as commcm 

 clay ; it consisted solely of comminuted corals and shells, with 

 colouring 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 detritus 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, the reefs 

 have probably added somewhat to these accumulations ; 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 anchored, 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 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 

 waters or channels, and the direction and force of the currents. 



