666 
GEOLOGY: R. A. DALY 
both factors is the manifest, special duty of all those who advocate the 
subsidence theory. 
Darwin stated that the fiUing of the concavity, here called a moat, 
is to be referred to ''the growth of the delicately branched corals within 
the reef, and to the accumulation of sediment" {The Structure and 
Distribution of Coral Reefs, London, 2d ed., 1874, p. 132). He did not 
precisely indicate the nature or origin of the sediment. In part it is 
derived from the erosion of the central island, though at a rate rapidly 
diminishing as the island sinks; in part from the reefs, inner and outer; 
and in part from the remains of non-coral organisms, both free-swimming 
and bottom species. The rain of planktonic and nektonic shells and 
skeletons to the bottom will, of itself, be nearly uniform in the lagoon 
and not directly competent to obliterate the moat, until the lagoon is 
filled almost to sea-level. Modern field studies, like those at Funa- 
futi, have shown that bottom lagoon organisms, including corals, are 
very largely restricted to depths less than 30 meters; that is, to shallows 
immediately adjoining the main reef, the coral knolls of the lagoon, 
and the shores of the central island, if present. In general the lagoon 
floor is 'sandy' and barren of growing coral. Dana held that the open- 
ocean waves throw into the lagoon, over the main reef, 'a large part' 
of the debris eroded from the growing, outer face of that reef. How- 
ever, the detritus thrown over the main reef by waves of the open 
ocean is practically nil in all those cases where the reefs form dry land; 
and there are evidences, not here to be detailed, that the relative 
amount of lagoon sediment so added to the deposits in the average 
lagoon is actually small. The sediment brought into the lagoon through 
the occasional ship channels may be more important, but most of it is 
deposited, in delta form, just inside the main reef. 
Thus, whatever be the various origins of the lagoon sediments, the 
detritus necessary to fill the hypothetical moat must be found princi- 
pally in the shallower places within the lagoon. If the detritus were as 
fine as mud, its transfer to the deeper part of the moat might be com- 
paratively rapid. Observation shows, on the other hand, that lagoon 
floors are characteristically more sandy than muddy, even in depths of 
50 meters or more. Marine sand is transported on the bottom. It 
follows, then, that, if the floor of the hypothetical moat were carried 
down by subsidence to depths greater than those where waves and 
currents can stir bottom sands, the moat could be obliterated only 
after the detrital shelves, advancing from the shallows, have become 
confluent. Obliteration of the moat by this process is extremely slow 
for narrow lagoons, still slower for lagoons 20 to 50 kilometers wide. 
