1898] THE BROADENING OF ATOLL-ISLETS 177 



ing his visit to the atoll of Mitiaro, Hervey Grouj), says, " Towards 

 midniuht we were alarmed by a strange rushing of waters under- 

 neath. Next day we were told that it was the strong influx of the 

 sea by a subterranean passage into the lagoon." ^ The same writer 

 has mentioned great submarine caverns in the reef, the haunts of 

 large sharks, which are pursued even to these recesses by the daring 

 Polynesian fishermen.- Such caverns form everywhere a conspicuous 

 feature in upheaved coral reefs. 



On Funafuti the coral on the margin of the reef flat appeared 

 to me to grow seawards in piers ; as these broaden their interstices 

 first form wide trenches, then narrow crevasses that may be stepped 

 across, which clefts tend to be roofed in by growth of NuUipores and 

 are narrowest at the surface, ultimately, proceeding inshore, they 

 become mere fissures and then disappear. This disappearance only 

 refers to the surface, for they probably form tunnels far into the 

 centre of the islet, as shown by the openings through which the sea 

 floods the mangrove swamp. 



Either these caverns and tunnels, shown by the foregoing quota- 

 tions to be a usual feature of coral reef growth, were excavated out 

 of the solid rock by the sea in some unknown way, which seems an 

 improbable hypothesis, or the caverns represent spaces enclosed by 

 growth of coral. On the latter supposition they must have been 

 so enclosed when the margin of the reef flat lay further and further 

 inland. Consequently they are being still enclosed as that margin 

 grows further and further seaward, and the shelf of reef flat, as was 

 sought to be proved, thus widens for the reception of successive 

 hurricane beaches. 



On the leeward side of an atoll the shingle beaches are absent, 

 their place being occupied by blown sand. The arrangement of this 

 material does not afford data of age and growth like that presented 

 by the shingle. Analogy will, however, suggest that the leeward 

 margin keeps pace with the peripheral extension of the atoll to wind- 

 ward, the spasmodic increase of the hurricane beaches being probably 

 represented by a gradual aggregation of blown sand. 



Summary. — The islets of an atoll have been shown by modern 

 investigators, especially by Guppy, to be increasing laterally, and 

 usually at a stationary level. These observations chiefly applied to 

 the inner or lagoon beaches. At the windward corner of an atoll 

 concretionary lines of shingle beach are frequently, perhaps usually, 

 arranged. Because each is the result of a cyclone, these are appro- 

 priately termed hurricane beaches. Like the rings shown in a 

 transverse section of a tree, these hurricane beaches may supply 

 chronological data. A continuation of beach building would bury 



1 Gill, "Life in the Southern Isles," 1876, p. 173. 



2 Gill, "Jottings from the Pacific," 1885, p. 149. 



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