ON CHANGES OF LEVEL IN THE PACIFIC OCEAN. 123 



of change ; amounting, however, as we have estimated, to a de- 

 pression of but two hundred and fifty or three hundred feet. 

 The northwestern islands of the group lie more within the coral 

 area, and correspondingly, they have wider reefs and channels, 

 and deep bays, indicating a greater amount of subsidence. 



Samoa. — The island of Upolu has extensive reefs, which, in 

 many parts, are three-fourths of a mile wide, but no inner chan- 

 nel. We have estimated the subsidence at one or two hundred 

 feet. The volcanic land west of Apia declines with an unbroken 

 gradual slope of one to three degrees beneath the sea. The ab- 

 sence of a low cliff is probable evidence of a depression, as has 

 been elsewhere shown. The island of Tutuila has abrupt shores, 

 deep bays and little coral. It appears probable, therefore, that it 

 has experienced a greater subsidence than Upolu. Yet the cen- 

 tral part of Upolu has very similar bays on the north, which 

 would afford apparently the same evidence ; and it is quite possi- 

 ble that the facts indicate a sinking which either preceded the 

 ejections that now cover the eastern and western extremities of 

 Upolu, or accompanied this change of level. Savaii has small 

 reefs, from which we gather no certain facts bearing on this sub- 

 ject. East of Tutuila is the coral island, Rose. It may be 

 therefore, that the greatest subsidence in the group was at its 

 eastern extremity. 



Feejee Islands. — We have already remarked upon this group. 

 A large amount of subsidence is indicated by the reefs in every 

 portion of the group, but it was greatest beyond doubt in the 

 northeastern part. 



Ladroncs. — The Ladrones appear to have undergone their 

 greatest subsidence at the north extremity of the range, the part 

 nearest the centre of the coral area: for although the fires at the 

 north have continued longest to burn, the islands are the smallest 

 of the group, the whole having disappeared except the summits, 

 which still eject cinders. The southern islands of the group 

 have wide reefs, but they afford no good evidence of any great 

 extent of subsidence since the reefs began to form. 



We have thus fo..owed around the borders of the coral area: 

 and besides proving the reality of the limits, have ascertained 

 some facts with reference to a gradual diminution of the subsi- 

 dence towards and beyond these limits. A line from Pitcairns to 

 Bird in the Hawaiian Group appears to have a corresponding po- 

 sition on the northeast with the southern boundary line of the 

 coral area ; the two include a large triangular area. An axis 

 nearly bisecting this triangular space, drawn from Pitcairns towards 

 Japan, actually passes through the region of greatest subsidence, 

 as we have before determined it, and may be considered the axial 

 line or line of greatest depression for the great area of subsidence. 



