GEOLOGY: W. G. FOYE 
309 
have destroyed the fragile undercut islets if the lagoon floors from which 
they rise were fashioned by this agency. 
The islands originated about the middle of the Tertiary period dur- 
ing the extrusion of the lavas of the second andesitic period of Viti 
Levu. The volcanoes thus formed were maturely eroded and subsided. 
Four or five hundred feet of coraliferous limestone were deposited un- 
conformably on the subsiding surfaces and were later elevated. A 
second period of erosion followed and again basaltic volcanoes built 
cones on the surfaces of the eroded limestone. Within quite recent 
times the islands have subsided 50 to 90 feet and the modern coral reefs 
are developing on the eroded and submerged platforms. 
4. Summary of the Theoretical Results of the Expedition. — My views 
concerning the origin of the barrier reefs and atolls of Fiji may be sum- 
marized as follows: — 
1. The elevated reefs were deposited in all known cases unconform- 
ably on eroded surfaces of volcanic rocks. Atolls and barrier reefs were 
among the forms developed during this period of subsidence. 
2. Many atolls and barrier reefs are now developing on platforms of 
elevated limestone which have been eroded to fairly even surfaces by 
atmospheric solution and later submerged. 
3. Of the several theories concerning the origin of barrier reefs and 
atolls only Darwin's theory postulates a subsiding surface eroded above 
sea-level. But this theory is firmly based on the conception of pro- 
gressive, though intermittent, subsidence of large segments of the earth's 
crust. If such subsidence is conceived to have begun in the early Ter- 
tiary period, the writer is convinced that it characterized the larger 
part of the Tertiary and Pleistocene periods in which the elevated lime- 
stones of Fiji developed. But since the Pleistocene period the algebraic 
sum of the movements has been positive and uplift has resulted, al- 
though the sum, if reckoned from the early Tertiary, is negative and 
the ultimate result has been subsidence. The present reefs are, however, 
dependent upon the Pleistocene and Recent movements for their form. 
Such movements have been progressively upward with only minor 
periods of subsidence. They are also local in their effect and are de- 
pendent undoubtedly on volcanic activity, in part due to the transfer 
of material from the inner to the outer portions of the earth's crust, and 
in part to the secular cooling and consolidation of the extruded lavas.^-'^ 
Hence it cannot be said that the modern reefs of Fiji fully support 
Darwin's theory. 
4. The data assembled by Daly^ and Vaughan^ convince the writer that 
Pleistocene platforms exist very generally throughout the coral seas. 
