J. D. Dana — Origin of Coral Reefs and Islands. 97 



ward through the Central Pacific, conforming well to the 

 suggestion of the Darwinian theory ; and corresponding in 

 direction approximately to the "axial line of greatest depres- 

 sion " referred to above — 'the line AA on the accompanying map. 



As regards the rest of the Central Pacific between the above 

 defined 3,000-4,000-f.athom 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.* 



11. Since a fringing reef is, by the theory of Darwin, the first 

 stage in the origin of an atoll, it was naturally regarded by him 

 as, in general, evidence of little or no subsidence, and even, at 

 times, of elevation. But since (1) bold shores are an occasion 

 for narrow reefs and for their absence; (2) submarine volcanic 

 disturbances and eruptions about volcanic lands would destroy 

 living reefs, or retard their progress where begun ; (3) islands 

 of active volcanoes have small or no reefs ; and (4) abrupt sub- 

 sidences of only 120 feet would put reef corals below a surviv- 

 ing depth and so lead to the beginning of a new reef, I was led 

 to regard the evidence from a fringing reef for no, or little, change 

 of level, as of very doubtful value. But the doubts, while 

 making such evidence generally useless, do not affect the value 

 of the preceding arguments for subsidence. Darwin used the 

 evidence from fringing reefs only to mark off the limits of the 

 area of Central-Pacific subsidence to which his coral-island 

 theory had led him ; and the same limits essentially are reached 

 notwithstanding the doubt. Instead of concluding that the 

 region along these limits was one of recent elevation or at least 



* 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-p W. and 177-J- W. (the " 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 sound- 

 ings, 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 (6,000 to 9,000 feet) on either side — suggesting the idea 

 of a central ridge. Over this plateau-area, there are, south of the Hawaian chain, 

 two or three small coral islands, and farther 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. Farther 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 Challenger soundings from the Sandwich Islands to Tahiti finding no 

 depth in the course of this belt greater than 2750 fathoms; but the belt on its 

 north side may continue eastward of the Challenger route. Many more lines of 

 soundings are needed to substitute sure conclusions for the above suggestions. 



The existence in the ocean of parallel belts of deeper and shallower waters, 

 such as are here inferred to exist, and such as are actually indicated by the parallel 

 lines of high islands and atolls, is in accordance with the facts over the continents. 



