358 CORALS AND CORAL ISLANDS. 
was general throughout the area. Moreover, each atoll, could 
we measure the thickness of the coral constituting it, would 
inform us nearly how much subsidence took place where it 
stands; for they are actually so many registers placed over 
the ocean, marking 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 the barrier reefs are, in general, 
evidence of less subsidence than atoll reets (p. 266). Conse- 
quently, the great preponderance of the former just below the 
southern boundary line of the coral island area, and, farther 
south, the entire absence of atolls, while atolls prevail so uni- 
versally north of this line, are evidence of some 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, 
gradually reduces its size until it finally becomes so small that 
the lagoon 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 about the equator, five or ten degrees south, 
between the Paumotus and the Gilbert Islands, are the small- 
est of the ocean; several of them are without lagoons, and 
some not a mile in diameter. At the same time, in the Pau- 
motus, and among the Gilbert 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 distance north of the boundary 
line, over the region of small equatorial islands, between the 
meridians of 150° and 180° W. 
c. When, after thus reducing the size of the atoll, the 
