366 On Coral Reefs and Islands. 
extends half*o two-thirds of amile 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 Samoan 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 eet. 
_ PART OF 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 réefs as there is no proper inuer 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 .cogal. At ‘['ongatabu, the bottom, where the , 
anchored, «was a grayish blue mud, appearing as plastic | 
“mon clay %é" eonsisted solely of comminuted coral and $ 
with coloring smatter probably from vegetable decomp 
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