160 J.D. Dana on Changes of Level in the Pacific Ocean. 
they are actually so many registers placed over the ocean, mark- 
ing out not only the site of a buried island, but also the depth at 
which it lies covered. We have not the means of applying the 
evidence ; but there are facts at hand, which may give at least 
comparative results. 
a. We observe, first, that barrier reefs are, in general, evidence 
of less subsidence than atoll reefs. (xiii, 186.) Consequently the 
great preponderance of the former just below the southern boun- 
dary line of the coral island area, and farther south the entire ab- 
sence of atolls, while atolls prevail so universally north of this 
line, are evidence of little depression just below the line; of less 
farther south ; and of the greatest amount, north of the line or 
over the coral area. 
b. The subsidence producing an atoll, when continued, gradu- 
ally reduces its size, until finally it becomes so small that the la- 
goon is obliterated ; and consequently a prevalence of these small 
islands is presumptive evidence of the greater subsidence. We 
observe, in application of this principle, that the coral islands 
abont the equator, five or ten degrees south, between the Paumo- 
tus and the Tarawan Islands, are the smallest of the ocean: sev- 
eral of them are without lagoons, and some not a mile in diame- 
ter. At the same time, in the Paumotus, and among the Tara- 
wan and Marshall Islands, there are atolls twenty to fifty miles 
in length, and rarely one less than three miles. It is probable, 
therefore, that the subsidence indicated was greatest at some dis- 
tance north of the boundary line, over the region of small equa- 
torial islands, between the meridian of 150° W. and 180°. 
c. When after thus reducing the size of the atoll, the subsi- 
dence continues its progress, or when it is too rapid for the grow- 
ing reef, it finally sinks the coral island, which, therefore, disap- 
pears from the ocean. Now it is aremarkable fact that while the 
islands about the equator above alluded to indicate greater subsl 
dence than farther south, north of these islands, that is, betw 
them and the Hawaiian Group, there is a wide blank of ocean 
in breadth. 
This area lies between the Hawaiian, the Fanning and the Mar- 
shall islands, and stretches off between the first and last of thesé 
groups, far to the northwest. ; 
$ it not then a legitimate conclusion that the subsident 
was least to the south beyond the boundary line, and increased 
northward, was still greater or more rapid over this opel area ; 
that the subsidence which reduced the size of the islands abot 
the equator to mere patches of reef, was farther continuec, 
caused the total disappearance of islands that once existed ove 
this part of the ocean ? i 
d. ‘That the subsidence gradually diminished southwest ger 
from some point of greatest depression situated to the north 
