CORAL REEFS AND SUBMARINE BANKS 217 
control theory, above cited, in which the following views are 
expressed : 
Nearly all of the oceanic islands and shoals seem to be of volcanic origin. 
Rising from a sea bottom 3,000 m. to 7,000 m. deep each volcano is very high 
in absolute measure and is also of notable area. The local extravasation of so 
much lava may well entail local, moderate sinking of the earth’s crust. It is 
indeed possible that such sinking is very often caused directly by volcanic 
action on a large scale... . . Possibly, therefore, some of the drowned valleys 
and other physiographic features showing submergence of volcanic islands are 
to be explained by local sinking to the extent of a few meters or a few scores 
of meters [233]. 
This appears to me a very reasonable statement of the case, 
except in the limitation of the resulting subsidence to “‘a few scores 
of meters,” that is, to only r or 2 per cent of the total altitude of a 
volcanic island that rises 2,000 meters over an ocean that is 6,000 
meters deep. If the subsidence be causally associated with the 
change of attitude of the huge volume of lavas required to build a 
high volcanic island, a subsidence amounting to ro or 20 per cent 
of the total height of the cone—that is, 1,000 or 2,000 meters— 
does not seem incredible. According to Molengraafi subsidence 
amounting to roo per cent of the total altitude of a volcanic island, 
measured from sea bottom up, is to be expected. However this 
may be, the possibility that the volcanic foundation of a coral reef 
may locally subside must greatly weaken the chain of arguments 
which go to prove that “the submarine physiography [of certain 
atolls] spells crustal stability rather than unrest,’ and hence that 
subsidence cannot have played an important part in atoll formation. 
Structural features of reefs formed according to the glacial-control 
theory.—The different classes of reefs, fringing, barrier, and atoll, 
are derived from each other, according to the theory of intermittent 
subsidence, as illustrated in Fig. 1 (p. 205); they are formed inde- 
pendently according to the glacial-control theory; and, as atolls are 
the most abundant and therefore receive chief consideration in 
Daly’s discussion, their structural features will be first examined 
here. They are in essence as follows: A preglacial volcanic island, 
UV, Fig. 2 (drawn on a larger scale than Fig. 1), is reduced, partly 
by subaérial erosion, partly by abrasion, during a long period of 
quiescence to an island of low relief, U'’W, while a bank, UB’, is 
