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



greatest depth obtained in the part of the area south-west of 

 the group (400 miles broad) is 3305 fathoms, and in the part 

 north-east 3448 fathoms. 



Again, the soundings of the 'Tuscarora' of 1875 here cited, 

 taken in connection with those of the same vessel in 1874, 

 under Commander George F. Belknap, along a line from the 

 Sandwich Islands westward to Japan (mostly between the 

 parallels of 20° and 25°), suggest the further conclusion, that 

 the deep-sea area of the central equatorial Pacific, in which 

 the Phoenix Islands stand, extends north-westward toward 

 Japan, and that it was crossed by the ' Tuscarora ' between 

 171° E. and 150° E., where were found depths from 3009 

 to 3273 fathoms (with some alternations of smaller depths 

 that isolated areas may account for). It is also probable 

 that the soundings of the c Challenger ' east of Japan be- 

 tween 153° E. and 143° E., and just north-west of those 

 of the ' Tuscarora/ were within the same deep-sea area. If 

 this be so, a long deep-water area or trough extends from 

 Japan south-eastward through the Central Pacific, conform- 

 ing well to the suggestion of the Darwinian theory; and 

 corresponding in direction approximately to the " axial line 

 of greatest depression " referred to above — the line AA on 

 the accompanying map. 



As regards the rest of the Central Pacific between the 

 above defined 3000-4000-fathom belt and the Hawaian chain, 

 the depths sounded by the ' Tuscarora ' are, with few exceptions, 

 over 2400 fathoms; two thirds of them are over 2750 fathoms; 

 and a fifth (out of the fifty-five in this area) over 3000 *. 



* The same two lines of soundings by the ( Tuscarora' suggest the 

 existence of a second long deep-sea belt or trough in the Central Pacific 

 just south of the Hawaian chain. This supposed trough was crossed by 

 this vessel in 1875 between the parallels of 13° N. and 18° N. (" Belknap 

 depths"), and in 1874 between the meridians of 172^° W. and 177^° W. 

 (the w Ammen depths ") ; the greatest depth found on the former line is 

 3125 fathoms, and on the latter 3106 fathoms. Should the existence of 

 these two troughs be sustained, the region between them would be a 

 Central- Pacific plateau ; having in it, along the 1875 line of soundings, 

 depths of 2972 to 1325 fathoms, and along that of 1874, depths of 2836 to 

 1108 fathoms ; the shallower portion is near the middle of each line of 

 soundings, has a great descent (6000 to 9000 feet) on either side, sug- 

 gesting the idea of a central ridge. Over this plateau-area there are, 

 south of the Hawaian chain, two or three small coral-islands; and 

 further eastward, the Palmyra, Kingman, Washington, Fanning, and 

 Christmas reefs and islands, which, although coral structures, make the 

 idea of a central ridge in this part for 600 miles almost a manifest fact. 

 Further east, the Marquesas Islands are in the same range. The deep belt 

 lying on the south side of the plateau diminishes in depth to the east- 

 ward, the i Challenger ' soundings from the Sandwich Islands to Tahiti 

 finding no depth in the course of this belt greater than 2750 fathoms; but 





