398 PACIFIC OCEAN. 
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 there is no inner channel. 
We have estimated the subsidence at one or two hundred feet. The 
volcanic land west of Apia declines beneath the sea with an unbroken 
gradual slope of one to three degrees. ‘The absence of a low cliff is 
probable evidence of a depression, as remarked on page 332. The 
island of Tutuila has abrupt shores, deep bays and little coral. It 
appears probable, therefore, that it has experienced a greater subsi- 
dence than Upolu. Yet the middle district of Upolu has very similar 
bays on the north (p. 312), which may be evidence of a similar sub- 
sidence; it is quite possible that the facts indicate a sinking which 
either preceded the ejections that now cover the eastern and western 
extremities of Upolu, or accompanied these eruptions. Savaii has 
small reefs, from which we gather no certain facts bearing on this 
subject. East of Tutuila is the coral island, Rose. It may be there- 
fore, 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 northeast part. 
Ladrones.—The Ladrones appear to have undergone their greatest 
subsidence at the north extremity of the range, the part nearest the 
axis of the coral area: for although the fires at the north have con- 
tinued longest to burn, the islands are the smallest of the group, the 
whole having disappeared except the summits which still eject cin- 
ders. The southern islands of the group have wide reefs, which afford 
evidence of little subsidence since the reefs began to form. 
We have thus followed 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 subsidence towards and 
beyond these limits. A line from Pitcairn’s to Bird in the Hawaiian 
Group (see chart) appears to have a corresponding position on the 
northeast with the southern boundary line of the atoll seas: it in- 
cludes a large triangular area. An axis bisecting this triangular 
space, drawn from Pitcairn towards Japan, actually passes through 
the region of greatest subsidence, as we have before determined it, and 
