W. M. Davis — Shaler Memorial Study of Coral Reefs. 271 



Mare - shows a small mass of volcanic rock, once an island in the 

 center of its lagoon, now a low hill rising over the elevated lagoon 

 plain; but the rim which represents the atoll reef around the 

 plain rises higher than the hill-top ; hence the volcanic island 

 was completely submerged before elevation took place. Uvea, 

 the northwestern of the three Loyalty islands, is a slightly tilted 

 atoll ; its eastern side shows an uplifted reef in crescentic 

 form, 100 or more feet high at the middle of its crescent, and 

 slowly descending to sea level at its horns ; a bight on the 

 convex eastern side may result from a land slide into the sea ; 

 the tilted lagoon floor slowly deepens westward and is enclosed 

 by disconnected, up-built reef -islands. 



The New Hebrides show signs of recent uplifts in their ele- 

 vated reefs, and of depressions in their embayments. There 

 is some evidence that certain uplifted fringing reefs on the 

 island of Efate, near the center of the group, were formed 

 during pauses in a subsidence that preceded their uplift, and 

 not during pauses in their uplift, as inferred by Mawson. 

 The narrowness of the lagoons enclosed by the barrier reefs 

 that encircle certain strongly embayed islands in this group 

 may be explained by supposing alternations of slow and rapid 

 subsidence, so that the earlier-formed reefs, which began to 

 grow when the subsidence was slowly initiated, were drowned 

 when it was later accelerated ; the new reefs thereupon begun 

 on the shoreline of that time would not now stand far outside 

 of the present shoreline, though the shoreline would be 

 strongly embayed because the total subsidence has been large. 

 The absence of reefs around the island of Ambrym is due to 

 its abundant eruptions in recent years, the latest one being in 

 December, 1913. Scattered corals were seen growing on one 

 of its sea-cliffed lava-streams, thus illustrating the initial stage 

 of a fringing reef. 



The Great Barrier reef of Australia, the largest reef in the 

 world, with a length of some 1200 miles and a lagoon from 15 

 to 70 or more miles wide, has grown upward during the recent 

 subsidence by which the Queensland coast has been elaborately 

 embayed, as was pointed out by Andrews in 1902. 



A few hours on shore at Raratonga, the southernmost mem- 

 ber of the Cook group, sufficed to show that extensive former 

 embayments entering its elaborately carved mass are now 

 occupied by delta plains and perhaps in part by slightly ele- 

 vated reef and lagoon limestones. 



Five islands of the Society group exhibit unequivocal signs 

 of recent subsidence in their intricately embayed shorelines, 

 as has lately been announced by Marshall. The cliff-rimmed 

 island of Tahiti, the largest and youngest of the group, has 

 suffered moderate subsidence after its cliffs were cut, but its 

 bays are now nearly all filled with delta plains ; hence a pause 

 or still-stand has followed its latest sinking. 



A.m.,Jour. Sci. — Foubth Series, Vol. XL, No. 237. — September, 1915. 

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