STRUCTURE OF CORAL REEFS. 



149 



lines. 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 



6 





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. 



