394 PACIPVC OCEAN: 
From the above review of evidences of change of level, it appears 
that where there are no barrier reefs, and only fringing reefs, the 
corals afford no evidence of subsidence. But it does not follow that the 
existence of only fringing reefs, or of no reefs at all, is proof against 
a subsidence having taken place. For we have elsewhere shown, 
(p. 134,) that through volcanic action, and, at times, other causes, 
corals may not have begun to grow till a recent period, and, therefore, 
we learn nothing from them as to what previously may have taken 
place. While, therefore, a distant barrier is evidence of change of 
level, we can draw no conclusion either one way or the other, as is 
done by Darwin, from the fact that the reefs are small or wholly 
wanting, until the possible operation of the several causes limiting 
their distribution has been duly considered. 
The influence of volcanoes in preventing the growth of zoophytes, 
extends only so far as the submarine action may heat the water, and 
it may, therefore, be confined within a few miles of a volcanic island, 
or to certain parts only of its shores. 
There are three epochs of changes in elevation which may be dis- 
tinguished and separately considered. 1. The subsidence indicated 
by atolls and barrier reefs. 2. Elevations during more recent periods, 
and also during the same epoch of subsidence. 3. Changes of level 
anterior to the atoll subsidence and the growth of recent corals. 
1. SUBSIDENCE INDICATED BY ATOLLS AND BARRIER REEFS. 
In a survey of the ocean the eye at once observes its numerous 
atolls, and sees in each, literally as well as poetically, a coral urn upon 
a rocky island that has sunk beneath the waves. Through the equa- 
torial latitudes, such marks of subsidence abound, from the Eastern 
Paumotu to the Western Carolines, a distance of about six thousand 
geographical miles. In the Paumotu Archipelago there are about 
eighty of these atolls. Going westward, a little to the north of west, 
they are found to dot the ocean at intervals, as our map is dotted with 
green; and at the Tarawan Group, the Carolines commence, in which 
are seventy or eighty atolls. 
If a line be drawn from Pitcairn’s Island, the southernmost of the 
Paumotus, by the Gambier Group, the north of the Society Group, 
