STRUCTURE OF CORAL REEFS. 



119 



bours ; 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, tlie reef is rather a fringing than a barrier reef. Within 

 the green belt that mcircles Bolabola (p. 109) 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 





CORAL REEFS OFF THE NORTH SHORE OF TAHITI. 



upper portions constitute the so-called barrier and fringing 

 reefs. The rock w^ould 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. 



Yet the bottoms of these channels are not always made up 

 of calcareous or coral sands and fragments ; for the volcanic or 

 basaltic lands they adjoin are a source of ordinary mud ; and 

 the river courses of the land and the tidal currents of the sea 

 will often determine the nature of the bottom, or may cause in 

 it alternate variations. 



