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



Hawaii, the easternmost, is mostly without reefs, as well as the 

 larger of the westernmost islands, Kauai, which has partly bold 

 and indented shores. 



13. To give a more adequate view of the changes of level, or 

 the evidences bearing on the subject, along the " limits" of the 

 central area of subsidence, I mention some additional facts. 



Tahiti is the large eastern island of the Tahitian Group. To 

 the westward, the islands (1) decrease in size ; (2) increase great- 

 ly in relative breadth of reef-grounds; (3) become deeply 

 indented in shores, as explained ; and (4) include an atoll, 

 Tubuai, as one of the last two of the chain. While the reef of 

 Tahiti proves little subsidence at that end of the group, and its 

 reefs and channels are extensive enough to make the proof 

 good, the other islands indicate, on the Darwinian theory, that the 

 subsidence increased much in amount westward. The western 

 end of the chain is about a degree nearer the equator than the 

 eastern. 



In the Samoan Islands, the largest island, Savaii, is the west- 

 ernmost; and from there the islands decrease in size eastward, 

 and end in an atoll, Rose Island. The group is like Tahiti in 

 gradation as to increase of subsidence, but the direction is the 

 reverse ; and this fact points apparently to a much deeper area 

 between them.* Moreover, although such broad barrier reefs as 

 those of Raiatea and Bolabola do not occur in the Samoan 

 group, bold shores do in Tutuila and Manua, and indicate the 

 participation of these islands in the subsidence, notwithstand- 

 ing their contracted reefs. Further, the reef of Upolu is 

 broad enough to be proof of little change in the region of that 

 island ; and there was little, probably, at Savaii, the larger 

 island west of it. The evidence of increased subsidence to the 

 eastward is strong, and narrowness of reef is no objection to it. 



At the Sandwich Islands the case is similar and yet different ; 

 similar in the fact that the largest island of the chain, Hawaii, 

 makes one of its extremities, the eastern, and a series of coral 

 islands the other — the whole length being 2,000 miles ; but 

 different in that no great reef exists about the shores of. either of 

 the eastern islands to prove that the subsidence there was small 

 or none. The elevated reefs are only a local phenomenon, and 

 do not prove the absence of subsidence during the era preced- 

 ing the elevation. 



But we have other evidence of importance derived from 

 soundings about the group by the Challenger in 1875 and the 

 Tuscarora in 1874, 1875. These soundings show that the deep- 

 sea area of 3,000 to 4,000 fathoms comes up quite closely about 

 the eastern end of the chain. It was found within 300 miles of 



* The distance between the remote extremities of these two groups is nearly 

 2,000 miles, and the interval between the nearer, over 800 miles. 



