30 EXPEDITION OF THE "ALBATROSS," 1899-1900. 



indicated by long lines of narrow islets thrown up on tlie reef platforms, 

 exactly as they are in the Pautnotus. These islands and islets are usnally 

 well wooded, and thus give a very peculiar aspect to the barrier reef. In 

 the case of Bora-Bora, Maupiti, and Aitutaki, for instance, we have a central 

 volcanic peak of considerable height surrounded by a wide lagoon, the sea 

 edge of which is formed by a fringe of wooded islets and islands forming a 

 more than half-closed ring aroinid the central island, which, in Bora-Bora 

 and Maupiti, rise in slopes and nearly vertical walls, the former to a height 

 of nearly 2400 feet, the other to about 800 feet. 



The only island of the Cook Group which we examined was Aitutaki ; 

 as Atiu is composed of elevated limestone, and Rarotonga is volcanic, I 

 hoped we might find that atoll to be in part volcanic and in part composed 

 of elevated coralliferous limestone ; we found it to be volcanic, an island 

 with the structure of Bora-Bora on a smaller scale. 



We anchored at Nine, an island composed of elevated coralliferous lime- 

 stone showing three well-marked terraces, the lowest of not more than five 

 to ten feet, and in many places disappearing completely, the limestone cliffs 

 rising vertically from the sea well into the second or even the third ter- 

 races. The vertical faces of the cliffs are dotted with caverns and deeply 

 indented by small cailons extending at right angles to the face of the 

 shore or forming blunt headlands separating short reaches of coral-sand 

 beaches. 



The second terrace varies in height from fifty to sixty feet, the third 

 from ninety to 100 feet. The second terrace is deeply undercut, and in 

 the higher vertical cliffs extending into the third terrace from the sea, the 

 former positions of the terraces are usually indicated by lines of caverns. 

 There are corals on the sea slopes of the first terrace, extending down to 

 ten or twelve fathoms, growing much as they are found at Makatea. 



From Nine we went to the Tongas, which we found a most interesting 

 group. The elevated Tertiary coralliferous limestones take here their great- 

 est development, and are on a scale far beyond that of their development in 

 the Lau Group of the Fijis, or the Paumotus. The first island of the Ton- 

 gas we visited, Eua, is perhaps the most interesting of the islands composed 

 of Tertiary elevated coralliferous limestone I have visited. From Dana's 



