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



breccia, and sometimes a very coarse breccia. Some masses of 

 it lying on the shore-platform of Paumotu atolls (thrown up by 

 storm or earthquake waves) 100 to 2000 cubic feet in contents, 

 consisted of single pieces of massive corals — Astrceas, Pontes, 

 etc. ; and others were an agglomeration of fragments of corals. 

 The fine-grained or impalpable kind made from coral mud 

 may have few or no fossils, and be a magnesian limestone. 



Another variety of the coral-reef rock, made in lagoons and 

 sheltered channels, has the corals in the position of growth, and 

 when formed of branching corals the spaces among the branches 

 are often but partly filled. It is a weak rock ; and the islets 

 thus made in lagoons and inner channels are sometimes over- 

 turned by the heaviest of waves ; and rising banks (as the ex- 

 perience of the "Wilkes Expedition proves) may be crushed be- 

 neath the keel of a passing vessel. 



Owing to the different modes of origin of the beach-made 

 rock and the true coral-reef rock, the occurrence of the former 

 underneath the latter would be evidence of subsidence. 



Deep borings in atolls with circular drills that would give a 

 six-inch core would supply evidence as to the existence or not 

 of beach-made coral rocks at levels below the surface. They 

 would also determine the depth to which true modern coral-reef 

 rock extends and the nature of the underlying beds, whether 

 calcareous, volcanic, or of any other kind ; this is hence a sure 

 method for obtaining a final decision of the coral island ques- 

 tion and should be tried.* 



19. JElevaied coral reefs afford an opportunity to search for 



* The "Wilkes Expedition carried out apparatus for boring. It was put into inex- 

 perienced hands, as Commodore Wilkes states in his Narrative (iv, 267, 268), and 

 at a trial with it on Aratica (Carlshoff Island) in the Paumotus, it became broken 

 and useless at a depth of 21 feet. Moreover, the granulated material brought up 

 afforded no satisfactory evidence as to the kind of coral rock encountered. 

 The statement in the Narrative that " the low coral islands, as far as they have been 

 investigated, both by boring and sounding, have shown a foundation of sand, or 

 what becomes so on being broken up," has been quoted and made more of than 

 the facts warrant. The "soundings" reached only the sands of the sea-bottom; 

 and the " boring," if it found sand at bottom, proved only that the beach-made rock 

 may exist at the 21 -foot level, in which case a small subsidence would be in- 

 dicated. 



Commodore Wilkes says on page 269 of the same volume: "The elevated coral 

 islands which we have examined exhibit a formation of conglomerate composed 

 of compact coral and dead shells, interspersed with various kinds of corals, which 

 have evidently been deposited after life has become extinct. A particular instance of 

 this was seen at the island of Metia, and the same formation was also observed at 

 Oahu." As the corals of a conglomerate, whether consisting of rounded masses 

 or angular, are " deposited after life has become extinct," no inference as to the 

 particular kind of coral rock intended can be drawn from the remark. From my 

 knowledge of the island I presume he meant the ordinary breccia conglomerate of 

 the reef rock, which is one of the kinds of coral rock of the elevated island. 

 Commodore Wilkes himself made no examination of the rock or special study of 

 coral islands, as might be inferred from his theoretical views on p. 270 of volume iv. 

 His Narrative was to a considerable extent made up from the Journals of his 

 various officers. 



