80 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the chart, for the whole of the visible part of the archipelago is included 

 in a line drawn east and west, south of Maka-pu ; south of that line the 

 position of the southwestern reef can be traced only by the discoloration 

 of the water. 



Manga Reva is an intermediate stage of erosion and denudation between 

 an arcliipelago lagoon such as Truk and a barrier reef island like Vanikoro, 

 and other islands in the Society group, as Bora Bora,^ Huaheine, Raiatea, 

 Eimeo, in which the surrounding platform has comparatively little width 

 and the barrier reef is close to the principal island and often becomes 

 a part of its fringing reef. INIanga Reva is open to the south and to 

 the west, Vanikoro to the east, while the volcanic islands of Truk are 

 completely surrounded by the outer encircling bai-rier reef, as are the 

 Society Islands just mentioned, which have several wide passages into 

 the lagoon through the wide barrier reef 



One is tempted to reconstruct the Gambler Archipelago of former times, 

 and to imagine it with a great central volcano, with a deep crater of 

 more than 34 fathoms, of which Manga Reva and Au Kena are parts 

 of the rim which once were connected from the southeast point of 

 Manga Reva to Au Kena, and thence along the line of the outer islets 

 to the northeast end of the former island. On the west face it was 

 flanked by smaller craters extending to the western islets of the barrier 

 reef of which the bays of Taku, Kirimiro, and Rumaru, and the bays of 

 the west side of Tara Vai are the eastern ridges. Tliere were probably 

 also other secondary volcanoes, of which Aka Maru and the islets of the 

 south part of the lagoon are the remnants, the latter all being situated 

 on the gentle slope of the southern part of the Manga Reva plateau ; 

 this may have been the southern slope of the principal volcano of the 

 group on the face of which have grown up the outer line of the barrier 

 reef and its islets. 



The existence of a central volcano with a deep crater would readily 

 explain the great depths of the lagoon in its different regions, and 

 oif the outer face of Manga Reva, depths showing slopes which are no 

 steeper nor more striking than the heights and slopes of tlie southern 

 part of Manga Reva, of Tara Vai, of Aka Maru, and of Maka-pu ; sup- 

 posing them to be extended into the sea. 



Mt. Mokoto and Mt. Dutf drop precipitously for more than one-third 

 their height, and in less than a quarter of a mile fall from over 1300 feet 

 to the level of the sea. Similar slopes are found along the volcanoes 



1 See A. Agassiz. Tlie Coral Reefs of the Tropical Pacific, Plates 210 and 

 231. 



