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GEOLOGY: W. M. DAVIS 
Darwin's theory remains, the simple sufficiency of which stands forth 
all the more clearly in contrast with the failing inefficiency of its 
competitors. 
The Problem of Atolls. The conversion of barrier reefs into atolls by 
a continuation of the process that has converted fringing reefs into 
barrier reefs is a highly probable matter; for it would be imreason- 
able to suppose that this process, whatever it is, should always have 
stopped before the central islands of barrier reefs were wholly sub- 
merged, and should never have worked in neighboring areas where 
reefs of identical form, but without a central island, are given another 
name. And as the converting process has, with so high a degree of 
probabihty, been shown to be subsidence of the ocean bottom in the 
region concerned, and not change of ocean level during the glacial period 
or uplift of the ocean bottom in some other region, it appears reason- 
able to explain atolls as well as barrier reefs by subsidence. This 
conclusion becomes all the more reasonable when the intimate asso- 
ciation of barrier reefs and atolls in the Fiji and Society groups and 
elsewhere is noted. Nevertheless the problem of atolls cannot be now 
regarded as absolutely solved, nor can it be absolutely solved until 
we make addition to our knowledge now undreamed of. 
Reefs and Reef -Platforms. A modification of Darwin's theory has 
lately been proposed by Vaughan, who regards recent submergence, 
proved by the embayments of the central islands, as the determining 
cause for the up-growth of existing barrier reefs, but who interprets 
the deeper and larger part of the entire reef-mass as an independent 
"platform" of earlier origin. As this investigator has not yet pubHshed 
his views regarding the origin of the reef -platforms, his modification 
of Darwin's theory will not be here discussed farther than to note that 
it seems inapplicable to many barrier reefs in the Fiji and Society groups; 
that the discontinuity of certain barrier reefs seems to be explicable 
on the assumption of imperfect up-growth during and after a recent 
and rapid subsidence, as well as on the assumption of independent 
origins for the reefs and their platforms; and that, while the extension 
of reef-platforms outside of the coral zone, as in the case of the Great 
Barrier reef of Australia, truly suggests a dual origin of reef -masses, 
this does not exclude the contemporaneous growth of platform and 
reef within the coral zone during long-continued but irregular or in- 
termittent subsidence. 
Summary of Results. The general result of my voyage, already an- 
nounced above, as well as several special results, may here be concisely 
stated: 
