Coral Reefs and Islands. 147 



another of like importance is added in my Report — the exist- 

 ence of deep fiord-like indentations in the rocky coasts of islands, 

 both of those inside of barriers and those not bordered by reefs. 



When making the ascent of Mount Aorai, one of the two 

 high summits of Tahiti (September 1839), where high narrow 

 ridges, almost like a knife-edge along their tops, alterna- 

 ting with gorge-like valleys 1000 to 3000 feet deep, radiate 

 from the central peaks but die out in a broad even plain at 

 the shores, I was made to appreciate the consequence to such 

 an eroded land of a partial submergence. At any level above 

 500 feet, its erosion-made valleys would produce deep bays, 

 and above 1000 feet fiord-like bays, with the ridges spreading 

 out in the water like spider's legs. Observing on the maps 

 of the Marquesas Islands precisely this condition, it was a 

 natural inference that the lands had undergone great subsi- 

 dence, and perhaps were still subsiding. 



With this criterion of subsidence in view, the evidence 

 from the Grambier and Hogoleu Islands is doubled in force ; 

 and that for the sinking of Eaiatea of the Tahiti group, re- 

 presented in fig. 3 of the plate of maps in Darwin's ' Coral- 

 Reefs,' is as strong from each of the two enclosed islands as 

 it is from the great breadth of the reef-grounds; and the same 

 is true for Bolabola, another Tahitian island on the plate. 



So it is also in the Feejees. The demonstration as to sub- 

 sidence in the large barrier-island called the Exploring Isles, 

 for example, is made complete by the form of the ridge of 

 land along one side of the great barrier, and the positions of 

 adjoining islets. 



5. The general parallelism between the trends of coral- 

 islands and the courses of the groups of which they are part, 

 and the courses also of the groups of high islands not far 

 distant, were regarded by Darwin as confirming his view 

 that the coral-islands were once high islands with bordering 

 reefs. 



6. Darwin uses also the argument that the large coral- 

 islands have the diversity of form found in the barrier-reefs 

 of high islands; and also that they often have such groupings 

 as would come from the sinking of a large island of ridges 

 and peaks with encircling reefs. He describes the Maldives 

 as one example of the latter, and the two loops of Menchikoff 

 island in the Caroline Archipelago as another. 



7. The depth of lagoons, and of the channels inside of large 

 barrier-reefs, afforded him further evidence of subsidence, it 

 being in many cases two to three times greater than the 

 limiting depth (120 feet) of living reef-making corals. 



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