ORIGIN OF BARRIER REEFS AND ATOLLS. 269 
and thus several atoll reefs may have resulted in place of the 
large one; and, further, each peak may have finally become 
the basis of a separate lagoon island, under a certain rate 
of subsidence or variations in it, provided the outer reef were 
so broken as to admit the influence of waves and winds. 
Some of the large atolls of the Maldives are properly atoll 
archipelagoes. 
The sizes of atolls offer no objection to these views, as 
they do not exceed those of many barrier reefs. 
All the conditions from fringing to barrier and from the 
barrier island to the atoll are admirably illustrated in the 
Louisiade Archipelago, Plate VII. The small amount of 
included high land within the enormous barrier, the linear 
form of the high islands, and the many islets which continue 
the line westward, the appendage-like relation to the large 
barrier-island of the islands at its northeast and northwest 
ends, look as if all the pieces of high land were parts of a 
nine-tenths-buried mountain-chain; and so much like it that 
any other supposition is evidently unreasonable. 
According to the principles explained and the facts illus- 
trating them, an atoll that 1s wooded through the larger part 
of its circuit, especially if not below medium size, bears evi- 
dence in this fact that the subsidence through which it was 
formed has probably ceased ; and on the contrary that atolls 
which are wholly or mostly covered with the sea at high tide, 
with few islets above high-water mark, are still undergoing 
subsidence. On this principle we may infer that the larger 
part of the Paumotus have passed to a period of cessation of 
subsidence, and that Keeling atoll in the Indian Ocean is of 
like character. Many of the northern Carolines, on the con- 
trary, may be still subsiding. 
It is of interest to follow still further the subsidence of a 
