14 



ON CORAL REEFS AND ISLANDS. 



extends half to two-thirds of a mile from the shore. Within 

 it, between Papieti and Matavai, there is an irregular ship chan- 

 nel, varying from three to twenty fathoms in depth. Occasionally 

 it enlarges into harbors ; and in other parts it is very intricate, 

 though throughout navigable by large vessels. The island of 

 Upolu, of the Samoati Group, is bordered by a reef nearly a mile 

 wide on part of its northern shore ; but the waters within are too 

 shallow for a canoe at low tide: and therefore, notwithstanding 

 its extent, the reef is rather a fringing than a barrier reef. 



l mile. 



3QgCjtS& 



PART OK THE NORTH SHORE OF TAHITI. 



The bottom of the channels or lagoons takes its charac- 

 ter, as regards the material constituting it, either from the reefs, a 

 source of calcareous sand and fragments, or from the earthy detri- 

 tus of the island streams. At Upolu, the white coral sand of the 

 reefs, (or in more general terms the reef debris,) forms the bottom ; 

 in some places it 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 conse- 

 quently no shore detritus mingled with the reef debris. At Ta- 

 hiti, the sounding lead usually brought up sand, shells, and frag- 

 ments of coral. At Tongatabu, the bottom, where the Peacock 

 anchored, was a grayish blue mud, appearing as plastic as com- 

 mon clay; it consisted solely of comminuted coral and shells, 

 with coloring matter probably from vegetable decomposition. 



