STRUCTURE OF CORAL REEFS. 149 
Jines. The outer reef extends half to two-thirds of a mile 
from the shore. Within it, between Papieti and Matavai, 
there is an irregular ship channel, varying from three to 




CORAL REEFS OFF THE NORTH SHORE OF TAHITI, 
twenty fathoms in depth. Occasionally it enlarges into har- 
bors; and in other parts it is very intricate, though throughout 
navigable by large vessels. The island of Upolu, of the Sa- 
moan 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 ex- 
tent, the reef is rather a fringing than a barrier reef. Within 
the green belt that encircles Bolabola (p. 138) there is a large 
and deep channel navigable by ships. 
Beneath these channels lies, in general, the coral rock of the 
reef-region—the inferior part of the great reef formation whose 
upper portions constitute the so-called barrier and fringing 
reefs. The rock would necessarily resemble that of the inner 
reefs already described; but there should be a larger propor- 
tion of the white compact limestone made from the fine coral 
sands carried off from the higher reefs by the currents. 
