J. D. Dana — Origin of Coral Reefs and Islands. 95 



These deductions have been apparently sustained by the 

 soundings of the Tuscarora and Challenger in 1874, 1875 and 

 1876. The soundings of the Tuscarora through the Phoenix 

 Group in 1875, on its route from the Sandwich Islands to the 

 Feejees (under the command of J. N". Miller, U. S. N., by the 

 order of the U. S. Hydrographic Bureau), are shown on the 

 map of the central Pacific herewith published (Plate II). 



The soundings about these islands prove (1) that the islands 

 are situated within the deep 3,000-4,000-fathom area of the 

 ocean ; and appear to indicate also (2) that along lines trans- 

 verse to the trend of the islands (or to the direction of trend in 

 other groups to the west), mean submarine slopes of 1:1*5 to 

 1 : 7 exist ; while in the direction of the trend, the slopes are 

 much less. The slope of 1 :T5, or that of the angle 38° 41', is 

 nearly the maximum slope of the sides of Cotopaxi, Mt. Shasta 

 and several other volcanic summits of Western America. 



The facts are these. 



Halfway between Sydney and Birnie's islands, 60 English 

 miles apart, a depth of 3000 fathoms (18,000 feet), was found. 

 Off Enderbury's Island (40 miles northeast of Birnie's), (1) a 

 depth of 2835 fathoms was obtained 20 miles to the southwest ; 

 (2) of 880 fathoms 2$ miles to the southwest ; (3) of 1991 

 fathoms 3 miles to the northeast; and (4) of 2370 fathoms, 11 

 miles to the northeast. The mean slopes to the southwest, cal- 

 culated from the soundings 1 and 2 are 1 : 6 and 1:3; and to 

 the northeast, from 3 and 4, 1 : 1*5 and 1 : 4, fourteen miles 

 southeast of Hull's Island, at right angles to the above direc- 

 tion, a depth of 935 fathoms was found, which gives the slope 

 1:13. 



Further evidence as to the submarine slopes about equatorial 

 coral-reef islands is afforded by soundings made under the di- 

 rection of the British Admiralty, near the very small Swain's 

 Island, at the south end of the Union Group (see map) ; and 

 others, by the Tuscarora under Commander Miller, in 1876, 

 near the Danger Islands, about five degrees east of Swain's. 

 Off Swain's Island, two soundings, one south of it and the other 

 east, (the two directions at right angles to one another and the 

 latter not diverging far from the trend of the other islands of 

 the Union Group), give the slopes 1 : 7 and 1 : 13. Off Danger 

 Island, as Commander Miller's Report states,* the depth of 660 

 fathoms was obtained half a mile (nautical) off the southwest cor- 

 ner of the reef near Southeast island, and 985 fathoms one mile 

 east of the reef, — corresponding to slopes 1 : 1 and 1 : 0*75. 1 : 1 is 

 a steeper slope than occurs even in small dry-made cinder cones ; 

 and 1 : 0'75 (53° 8') is steeper still. 



* I am indebted for the soundings about Danger Islands to Commander J. R. 

 Bartlett, Superintendent of the TJ. S. Hydrographical Bureau. 



