94: J. D. Dana — Origin of Coral Beefs and Islands. 



of 180°, are, with one exception (Canton or Mary), not over 

 four miles long. The three more southern of the Phcenix 

 Islands (see Map, Plate II), Gardner's, Hull's and Sydney, 

 between 4° 25' S. and 4° 35' S., are two to four miles 

 long and have lagoons ; five, including Phcenix, Birnie's and 

 Kean's, between 3° 10' S. and 3° 30' S., and Rowland and 

 Baker's north of the equator, are a mile and a half and less 

 in length, and have depressions at centre but no lagoons. The 

 depressions contain guano, and one of them, Kean's, has much 

 gypsum mixed with the guano ;* Kean's and Phoenix have a 

 foot or two of water at high tide, the tide rising 6 feet. Another 

 of the number, Enderbury's, is three miles long and has a half- 

 dried lagoon which is very shallow and has no growing corals. f 

 To the north of these islands for fifteen degrees of latitude, the 

 sea is an open one, and in the next ten degrees, to the line of 

 the Hawaian Chain, the only islets not marked doubtful are 

 "Coral Reef, Awash " and Johnston Island. 



A similar gradation in size takes place in the Ellice, Ratack 

 and many other groups of the ocean. Smallness of size, and 

 dried lagoon basins, with occasionally a deposit of gypsum from 

 evaporated sea-water, are just the result that should have come, 

 by the Darwinian theory, from subsidence ; and gradation in 

 size from gradation in the amount of subsidence. The positions 

 of the Union, Gilbert, Ratack and Ralick Groups with reference 

 to the Phcenix Group are shown on the Map, Plate II. All of 

 the islands on the map are coral islands, and nearly all atolls ; 

 and the part of the encircling reef marked by fine dots is 

 under water at high tide. 



Adopting this view of the origin of these smallest of coral- 

 made islands, I readily accepted Darwin's second conclusion 

 as to a great central oceanic area of subsidence. The further 

 inference, also, was deduced, for reasons stated in my Report, 

 that the greatest amount of subsidence took place along a belt 

 stretching southeastward from the southern half of Japan and 

 passing south of the Marquesas Group toward Easter Island, 

 and a line was drawn on a map among its illustrations represent- 

 ing the course of " the axial line of greatest depression."^: 



* J. D. Hague, Amer. Jour. Sci., II, xxxiv, 242. Mr. Hague, in his valuable 

 paper on the Guano Islands of the Central Pacific, mentions the existence of a bed 

 of gypsum two feet thick under the guano of Jarvis Island, another small equa- 

 torial island, eleven degrees east of the Phoenix Group. 



f Baker's Island has a height of 22 feet according to Mr. Hague, showing, he 

 says, some evidence of elevation; and Enderbury's I found to be 18 feet in 

 height, from which I inferred some elevation. But Howland's, Birnie's, McKean's, 

 Phoenix, Gardner's, Hull's aDd Sydney are not higher than ordinary atolls would 

 be in a sea of 6-foot tide. 



The facts with regard to the "Reef" on the map, in long. 175° W. and lat. 

 2° 40' S., I have been unable to learn. 



\ Report, pp. 399 and 432, and map between pages 8 and 9. 



This line is reproduced on a chart of the "World, in my Manual of Geology, 

 where it is lettered A' A'. 



