Febeuary 23, 1900.] 



SCIENCE. 



289 



excellent charts of the Society Islands, so 

 that it was comparatively simple to examine 

 the typical points of the group and to gain 

 an idea of their structure as far as it relates 

 to coral reefs. The Society Islands are all 

 volcanic islands edged with shore plat- 

 forms, some of great width, upon which the 

 barrier or fringing reefs of the islands have 

 grown. The structure of the reefs of the 

 Society Islands is very similar to that of 

 the Fiji reefs round volcanic islands. A 

 comparison, for instance, of the charts of 

 Kandavu, Viti Levu, Mbengha, Nairai, and 

 and of other volcanic islands in the Fijis, 

 with those of the Society group, will at 

 once show their identity. Huge platforms 

 of submarine denudation and erosion char- 

 acterize both, with fringing and barrier 

 reefs determined by local conditions. Per- 

 haps it is easier to follow the changes which 

 have taken place in the Society Islands ; 

 and such islands as Tahaa and Bora-Bora, 

 where we anchored, as well as Maupiti, are 

 admirable examples and epitomes of the 

 structure and mode of formation of the 

 coral reefs of that group. 



In Motu Iti and Tetuora the volcanic 

 peaks have disappeared, leaving nothing 

 but a shallow platform upon the outer 

 edges of which sandy coral islets have been 

 thrown up. There is, however, one point 

 in which the barrier reefs of the Society Is- 

 lands differ from those of Fiji. The barrier 

 reefs in Fiji are generally indicated merely 

 by reef flats, upon which the sea breaks, 

 and an occasional rocky islet or negro-head; 

 only rarely do we find sand keys upon the 

 fringing reefs of the islands of Fiji. In the 

 Society Islands, on the contrary, we find the 

 line of the barrier reef usually well indi- 

 cated by long lines of narrow islets thrown 

 upon the reef platforms, exactly as they are 

 in the Paumotus. These islands and islets 

 are usually well wooded, and thus give a 

 Tery peculiar aspect to the barrier reef. In 

 tlie case of Bora- Bora, Maupiti and Aitu- 



taki, for instance, we have a central vol- 

 canic peak of considerable height sur- 

 rounded 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 around 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 

 about 1100 feet. 



The only island of the Cook group which 

 we examined was Aitutaki, as Atiu is com- 

 posed of elevated limestone, and Earotonga 

 is volcanic. I hoped we might find that 

 atoll to be in part volcanic and in part com- 

 posed of elevated coralliferous limestone ; 

 we found it to be volcanic, an island with 

 the sti'ucture of Bora- Bora on a smaller 

 scale. 



"We anchored at Niue, an island composed 

 of elevated coralliferous limestone showing 

 three well-marked terraces, the lowest of 

 not more than 5 to 10 feet and in many 

 places disappearing completely, the lime- 

 stone clifis 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 caiions extended at right angles to 

 the face of the shore or forming blunt head- 

 lands separating short reaches of coral 

 sand beaches. The second terrace varies in 

 height from 50 to 60 feet, the third from 90 

 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 to 10 or 12 fathoms grow- 

 ing much as they are found at Makatea. 



From ISTiue we went to the Tongas, which 

 we found a most interesting group. The 

 elevated tertiary coralliferous limestones 

 take here their greatest development, and 

 ai-e on a scale far beyond that of their 



