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



Japan and passing south of the Marquesas group toward 

 Easter Island, and a line was drawn on a map among its 

 illustrations representing the course of " the axial line of 

 greatest depression " *. 



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 pub- 

 lished (Plate I.). 



The soundings about these islands prove (1) that the 

 islands are situated within the deep 3000-4 000-f a thorn area 

 of the ocean; and appear to indicate also (2) that along lines 

 transverse 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 : 1*5, or that of the 

 angle S3 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 north-east of 

 Birnie's), (1) a depth of 2835 fathoms was obtained 20 miles to 

 the south-west; (2) of 880 fathoms 2| miles to the south-west; 

 (3) of 1991 fathoms 3 miles to the north-east ; and (4) of 

 2370 fathoms, 11 miles to the north-east. The mean slopes 

 to the south-west, calculated from the soundings 1 and 2, are 

 1:6 and 1:3; and to the north-east, from 3 and 4, 1 : 1*5 and 

 1:4; 14 miles south-east of Hull's Island, at right angles 

 to the above direction, a depth of 935 fathoms was found, 

 which gives the slope 1: 13. 



Further evidence as to the submarine slopes about equa- 

 torial coral-reef islands is afforded by soundings, made under 

 the direction 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 



* .Report, pp. odd and 43'2, 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'. 



