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January 19, 1900.] 



SCIENCE. 



95 



The appearance of the old ledge and of 

 the modern reef rock is so strikingly differ- 

 ent that it is very simple to distinguish the 

 two, even when only comparatively small 

 fragments are found. 



We did not find in the Paumotus, as in 

 Fiji, all possible stages of denudation and 

 of submarine erosion between islands like 

 Vatu Vara, Niau, Kambara, Fulanga, 

 Ongea, Oneata, Ngele Levu, and Weilang- 

 ilala, and atolls with a mere ring or surf to 

 indicate their existence. 



In the Paumotus the islands have been 

 elevated to a very moderate height and 

 probably to nearly the same height, for the 

 old ledge forming the base of the modern 

 structure is found exposed nearly every- 

 where at about low-water when it cannot 

 be traced at a slightly greater elevation. 

 This would readily account for the nearly 

 uniform height of the islands throughout 

 the group. 



But there is another element which comes 

 into play in this group, and has an impor- 

 tant part in shaping the ultimate condition 

 of these atolls. At the Fijis we have seen 

 the submarine erosion continue until there 

 is little left of many of the atolls beyond 

 the merest small islet or rock to indicate 

 its structure. In the Paumotus, in the 

 great atolls which are evidently only the 

 exposed summits of parts of ridges or spurs 

 of an extensive tertiary coralliferous lime- 

 stone bed, the rim of the atoll is, after having 

 been denuded to the level of the sea, again 

 built up from the material of its two faces 

 which is thrown up on the wide reef flats 

 both from the sea face and from the lagoon 

 side. We do not find in the Fijis such 

 huge reef shelves to supply such masses of 

 material from the breaking up of the outer 

 and inner edges of the tertiary limestone 

 platforms, in addition to the fragments of 

 recent corals growing upon the flat and its 

 edges, which, when dead, are thrown up 

 and formed into shingle and sand to form 



a pudding stone, or a conglomerate, or 

 breccia, with the fragments of the old ledge 

 on the top of the reef flats. 



This pudding stone, or beach rock, is 

 found on all the reef flats of the islands of 

 the group. It forms great bars, at right 

 angles usually to the shore-line, and upon 

 the sea-face of these bars is thrown up coral 

 shingle, both old and recent, which builds 

 up short reaches of beaches separated by 

 wide flats through which the sea rushes at 

 high water, or merely covers the fiats at 

 low tide ; while on the lagoon side of the 

 wide reef flats a similar process is going 

 on, throwing up finer sand among the 

 beach-rock bars and along their sides, and 

 thus building up, little by little, at first 

 small sand bars, then larger bars, or islets, 

 at right angles to the shore-line, and as 

 they become larger by accretions from both 

 sides, they finally form an island from 

 1000 to 1200 feet long, according to the 

 width of the reef flat, extending from the 

 lagoon edge of the flat to the sea face of the 

 atoll. The sand bars, little by little, be- 

 come covered with vegetation, and at some 

 stages of tide appear like islands and islets 

 situated a considerable distance within the 

 lagoon. Whenever the material supplied 

 both from the lagoon side and from the 

 sea face is very abundant, the land ring 

 becomes more or less solid, the islets be- 

 come consolidated into islands, separated 

 by narrow or wider cuts, until finally they 

 form the larger islands which seem at first 

 glance to form continuous land along the 

 rim of the lagoon, but which are often 

 seen to be separated according to local 

 conditions by narrow cuts, which finally 

 allow no water to pass thi'ough and merely 

 indicate the former separation of the vari- 

 ous parts of the land. 



In the lagoons of atolls of such great 

 length as some of these of the Paumotus, 

 like Rairoa, Fakarava, Makemo and Hao, 

 which are between 30 and 40 miles long, 



