20 "ALBATROSS" TEOPICAL PACIFIC EXPEDITION. 



days in both these atolls. We usually timed our visits to the islands where 

 we could not anchor so as to spend the day, or the greater part of the day, 

 at these atolls, making our passages at night, and sounding whenever prac- 

 ticable on the way. 



All the islands we examined are, without exception, formed of tertiary 

 coralliferous limestone, which has been elevated to a greater or less extent 

 above the level of the sea, and then planed down by atmospheric agencies 

 and submarine erosion, the greatest elevation being at Makatea (about 

 230 feet), and at Niau, where the tertiary coralliferous limestone does 

 not rise to a greater height than 20 feet. At Rangiroa it was 15 

 to 16 feet high. At other islands it could be traced only as forming 

 the shore platform, from 50 to 250 feet wide, which forms the outer face 

 of the Paumotus and is so characteristic a feature of the atolls of the group. 

 [n other parts the old ledge could be traced cropping up in the interior of 

 the outer land rim, or in the open cuts connecting the lagoon with the outer 

 sea face of the atolls. Everywhere the space between the outcroppings of 

 the old ledge, as I will call the tertiary coralliferous limestone, was filled 

 with beach rock, or a pudding-stone, or with a breccia or conglomerate of 

 coralliferous material consisting in part of fragments of the old ledge, and 

 of fragments of recent corals and shells cemented together. The appear- 

 ance of the old ledge and of the modern reef rock is so strikingly different 

 that it is very simple to distinguish the two, even where only comparatively 

 small fragments are found. 



We did not find in the Paumotus, as in Fiji, all possible stages of denu- 

 dation and of submarine erosion between islands like Vatu Vara, Naiau, 

 Kambara, Fulanga, Ongea, Oneata, Ngele Levu, Wailangilala, and atolls 

 with a mere ring of surf to indicate their existence. In the Paumotus 

 nearly all the islands have been elevated to a very moderate height and 

 probably to about the same height, for the old ledge forming the base of 

 the modern structure' is found exposed nearly everywhere at about low 

 water, when it cannot be traced at a slightly greater elevation. This would 

 readily account for the uniform height of the islands throughout the group. 



But there is another element which comes into play in this group, and 

 has an important part in shaping the ultimate condition of these atolls. At 



