92 J. D. Dana — Origin of Coral Reefs and Islands. 



Bacon's Isles are the last two little peaks of a still larger 

 lagoon ; and besides these and other examples of disappearing 

 lands, a dozen of the easternmost islands are actual atolls — the 

 last peak gone. 



4. To this the chief of Darwin's arguments in his own view, 

 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, alternating 

 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 subsidence, and 

 perhaps were still subsiding. 



With this criterion of subsidence in view, the evidence from 

 the Gambier and Hogoleu islands is doubled in force : and that 

 for the sinking of Raiatea of the Tahiti group, represented 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 subsi- 

 dence 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 adjoin- 

 ing islets. 



5. The general parallelism between the trends of coral islands 

 and the courses of the groups of which they are a 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 larger 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 ex- 

 ample 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 



