216 W. M. DAVIS 
subsidence which gives them opportunity for upgrowth is locally 
associated with their volcanic foundation or is broadly manifested 
over large oceanic areas. In so far, however, as subsidence is 
associated with volcanic cones it is interesting to note that, as a 
result of the construction of many volcanic islands by long- 
continued eruptions through parts of Tertiary and later time, and 
as a further result of reef upgrowth upon such islands when they 
subside after their eruptive growth ceases, the ocean would be 
somewhat raised above the level that it had before the eruptions 
began; and thus the objection sometimes urged that the theory of 
subsidence is inconsistent with the prevalence of embayed conti- 
nental coasts would be removed... 
In support of the possibility of the local subsidence of volcanic 
reef foundations attention may be called to two recent articles. 
One is by Molengraaff,t in which the subsidence of an oceanic 
volcanic cone is causally connected with the elevation of its heavy 
lavas from a lower to a higher position, whereby the equilibrium 
of the ocean-floor crust is disturbed. It should be noted that 
oceanic volcanoes are more likely to cause isostatic subsidence than 
continental volcanoes, because the erupted materials of the latter 
are worn down and distributed far and wide in a relatively short 
geological period, while the materials of the former remain upon 
the site of their eruption indefinitely; indeed, instead of suffering 
loss by erosion, oceanic volcanoes enjoy a gain of volume by the 
addition of coral reefs if they stand in the warmer oceans, and it 
would therefore seem that the addition of a large volume of reef 
limestone to an oceanic cone would increase its tendency to subside.’ 
The atoll stage of reef development may therefore be a late phase 
in a normal series of changes. It is not, however, intended to 
state that changes in the ocean bottom take place only in con- 
nection with volcanic islands; there is abundant evidence to the 
contrary. 
The other article here to be cited in favor of the sub- 
sidence of volcanic islands is Daly’s exposition of the glacial- 
1G. A. F. Molengraaff, ‘‘Het Probleem der Koraaleilanden en de Isostasie,” 
Proc. k. Akad. Wet. Amsterdam, XXV (1916), 215-31. 
2“‘The Isostatic Subsidence of Volcanic Islands,’ Proc. Nat. Acad. Sci., II 
(1917), 649-54. 
