GEOLOGY: R. A. DALY 
665 
until the deepest part of each moat has sunk scores, hundreds, or pos- 
sibly thousands, of meters (see fig. 1). 
The observed general flatness of the lagoon floors of atolls shows that 
there the hypothetical moats must have been completely filled and obht- 
erated as distinct submarine forms. The depression occupied by the 
lagoon water inside each barrier reef has the position of the hypothetical 
moat, but in every case the lagoon floor is about as flat as an atoll- 
lagoon floor. For each kind of lagoon, therefore, the filling of the moat 
c 
o I 2 «■ s JO o 5 io_ 
5eo Miles Kilometers 
FIG. 1 
Sections illustrating the subsidence theory as applied to the wider barrier and atoll la- 
goons; also illustrating the relations of moat, moat filling, lagoon, and central island. There 
is no exaggeration of the vertical element, and the depth of water in the deepest existing 
lagoons is no greater than that shown, to scale, by the width of the black line drawn below 
the word 'Lagoon.' Water indicated by solid black. 
A. North-south section of the island of Kauai, chosen as a large, deeply eroded, volcanic 
island, which may be fairly presumed to typify the ancestor of one of the larger atolls, ac- 
cording to the subsidence theory. Fringing reefs indicated. 
B. Section of the same island after subsidence, with the growth of each fringing reef 
(now become a barrier) upward and outward on its own talus {T). Between the reef and 
central island is the moat, the upper part of which is occupied by the water of the lagoon. 
The detrital filling of the moat is not shown. 
C. Section of the same island after further drowning, with widening of the moat and 
ultimate development of an atoll. 
must be assumed to be far advanced. The inferred maximum depths 
of calcareous detritus are to be estimated in scores, hundreds, or pos- 
sibly thousands, of meters. The mechanism of this filling has never 
been adequately discussed; yet its analysis cannot fail to indicate a 
valuable test of the subsidence theory of reefs. To the writer's knowl- 
edge, no earlier statement of the test has been pubHshed. 
The filling mechanism involves two factors: sediment, and active 
transportation of that sediment. Neither has been thoroughly dis- 
cussed by Darwin or by any of his supporters; yet serious attention lo 
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