APPENDIX. 753 



stone, usually a little friable and in some parts becoming breeeiated. It forms 

 just above the water-level, and also below tbe surface to a depth of several — per- 

 haps many — feet. This variety of rock is uncommon among the extensive reefs 

 of the Pacific, the kind most approaching it being a sea-shore rock made from 

 coral sands. The rock there formed from coral mud or sand at moderate depths 

 in the lagoon and off shore is a white, compact, unfossiliferous limestone, having 

 the flint-like fracture of the Bird's-eye limestone (of the Trenton period) in 

 central New York (p. 206). 



The origin of the long curving line of coral reef stretching southwestward 

 from southern Florida to the Tortugas, and having a total length of 120 miles, 

 has been satisfactorily explained by Captain Hunt, who attributes the prolon- 

 gation to the transportation of coral sands from the coral reef to the eastward 

 by the Labrador current (p. 657). He shows that the current is just such as 

 would produce the form presented by the reef. This barrier-reef has, therefore, 

 the same origin as those of siliceous sands farther north on the American coast. 

 The only difference is this : that the material of which the sand of the coral reef 

 is made is a result of the growth of animal life on an earlier part of the reef. 

 The sound between the reef and Florida is about 120 feet deep, and has a bottom 

 of clean coral sand, the material of which, as Captain Hunt shows, is washed 

 from the reef. It is still possible that a subsidence has aided in producing the 

 results. 



3. Soundings near coral islands. — Among the Paumotus, according to Wilkes, 

 southeast of Ahii, the lead struck at 150 fathoms, and then fell off, and finally 

 brought up at 300 fathoms; 2 miles east of Serle's Island, no bottom was found 

 at 600 fathoms; li miles south of the larger Disappointment Island, no bottom 

 at 550 fathoms; a mile from the east end of Metia, no bottom at 600 fathoms. 

 Off Whitsunday Island, Beechey found no bottom at 250 fathoms. Darwin 

 states that Lieut. Powell found, 600 feet from Diego Garcia, no bottom with 150 

 fathoms; that at Cardoo Atoll Island, 300 feet off, no bottom was obtained at 

 200 fathoms; 2200 yards from Keeling Island, Fitzroy found no bottom at 1200 

 fathoms; but the line at a depth between 500 and 600 fathoms was partly cut, 

 as if it had rubbed against a projecting ledge of rock. 



These facts bear on the question of the thickness of coral reefs. 



4. Chalk. — The only locality of chalk among the Pacific coral-reef rocks, ob- 

 served by the author, occurs on the island of Oahu (Hawaian group). It has 

 but a few yards of extent, and is situated close by an injected dike of lava; 

 and it is probable that it owes its origin in some way to heat connected with 

 the injection. The author regards it, therefore, as no true exception to the state- 

 ment made on page 617 that chalk is not one of the varieties of coral rock: it 

 is made by accumulations of Rhizopod shells, and not of coral or shell sand. 



F. — Progress of Life (pp. 596, 597). 



Dividing the Animal kingdom into Invertebrates and Vertebrates as the 

 two subdivisions, the Cephalopods (Orthocerata, etc.) may perhaps be regarded 

 as the comprehensive type, foreshadowing the latter, as explained on page 

 397. 



The internal bone in Cephalopods, mentioned on p. 397, exists only in those 



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