GEOLOGY: W. M. DAVIS 
471 
for a second time. It is interesting to note that the upheaval of the 
Exploring Isles, by which the previously extinguished Trigger reef be- 
came resurgent, was of a significantly earher date than the resurgence 
of Frost reef between Mango and Vatu Vara; for the limestones of 
these two islands are httle dissected, while those of the Exploring Isles 
have been eroded to mere remnants of their former volume. It is the 
present almost-extinction of Trigger rock in a renewed subsidence of 
its region that is contemporaneous with the resurgence of Frost reef. 
Darwin's theory of intermittent subsidence is the only one of many 
coral-reef theories, which can account for the facts here adduced. 
THE ORIGIN OF CERTAIN FIJI ATOLLS 
By W. M. Davis 
DEPARTMENT OF GEOLOGY AND GEOGRAPHY. HARVARD UNIVERSITY 
Received by the Academy, June 22, 1916 
The unconformable contact of many fringing reefs on the eroded 
submarine slopes of oceanic islands, and the embayment of such islands 
inside of barrier reefs are inconsistent with certain modern theories of 
coral reefs ; and the recognition of these significant features has in recent 
years led several observers back to Darwin's earlier theory of upgrowth 
on intermittently subsiding foundations. My own experience two years 
ago, while on a Shaler Memorial voyage across the Pacific con- 
cerning which a brief report has been published in these Proceedings 
(1, 1915, 146-152), added evidence of reef formation during two 
periods of subsidence; the first subsidence being shown by the occur- 
rence of elevated reef limestones resting unconformably on eroded vol- 
canic foundations, as seen in Vanua Mbalavu and Avea, two of the 
reef-encircled cluster of the nine Exploring Isles in the eastern part of 
the Fiji group, the second subsidence being shown by the embayment 
of these now-dissected limestones, around which a new barrier reef 
has grown up, as stated more fully in the American Journal of Science 
for September, 1915. 
Continued attention to this problem has lately enabled me to per- 
ceive that the evidence of two periods of subsidence and reef growth 
found in the Exploring Isles may be extended to several neighboring 
atolls, the area concerned being well shown in Plate 19 of Agassiz' 
''Islands and Coral Reefs of Fiji" {Bull. Mus. Comp. Zool.^ xxxiii, 1899): 
thus singularly enough a reconcihation is permitted between Agassiz' 
theory of the formation of atolls on uplifted and worn-down limestone 
islands and Darwin's theory of the formation of atolls by upgrowth on 
