160 Dr. J. D. Dana on the Origin of 



coral-reef rock is of under-icater consolidation, and is usually 

 very compact, like an ordinary limestone ; and if a conglo- 

 merate, it is commonly a breccia, and sometimes a very coarse 

 breccia. Some masses of it lying on the shore-platform of 

 Paumatu atolls (thrown up by storm or earthquake-waves), 

 100 to 2000 cubic feet in contents, consisted of single pieces 

 of massive corals — Astrceas, Porites, &c. ; 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 some- 

 times overturned by the heaviest of w T aves ; and rising banks 

 (as the experience of the Wilkes Expedition proves) may be 

 crushed beneath 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 6-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 question, and should be tried *. 



* The Wilkes Expedition carried out apparatus for boring. It was put 

 into inexperienced hands, as Commodore Wilkes states in his i Narrative' 

 (iv. pp. 267, 268), and at a trial with it on Aratica (Garish off 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 

 i Narrative ' that " the low coral-islands, as far as they have been inves- 

 tigated, both by boriug 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 indicated. 



Commodore Wilkes says on p. 269 of the same volume : — "The elevated 

 coral-islands which we have examined exhibit a formation of conglome- 

 rate 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.' 1 As the corals of a 



