GEOLOGY: W. M. DAVIS 
147 
Theories of Coral Reefs. Before setting out on the voyage I reviewed 
the various theories of coral reefs in an essay that was pubHshed during 
my absence under the title of ''The Home Study of Coral Reefs" in the 
Bulletin of the American Geographical Society for 1914. Every one of 
the several theories is successful in explaining the visible features of 
the reefs themselves, provided the postulated conditions and the invis- 
ible processes of the past are accepted. Evidently then a study of 
the reefs alone will not suffice to discover which theory really provides 
a correct mental counterpart of their past and unobservable history. 
Hence appeal must be made from the non-committal reefs to competent 
witnesses of some other kind, which were present while the reefs were 
forming and \Thich are willing to testify about the events which then 
took place. 
Evidence derived from Barrier Reefs. In searching for such witnesses 
it should be borne in mind, first, that as far as fringing reefs A, A, A, 
figure 1, are concerned, their origin is hardly in debate; they are grow- 
ing colonies of corals, initiated by the arrival from elsewhere of pas- 
sively floating larvae, which establish themselves in shallow water close 
to a newly offered and suitable shoreline; second, that as far as atolls 
— or reef-rings enclosing shallow lagoons without central islands — are 
concerned, they are, unless penetrated by numerous and expensive 
borings, inscrutable, for they stand alone and bury their past; third, 
that as far as elevated reefs are concerned, their inner structure and 
their relation to the foundation on which they were formed would give 
important evidence regarding their origin, and should therefore be 
investigated; and fourth, that it is in connection with barrier reefs, 
B, B, B, figure 1, that the desired witnesses to the facts of the past 
can be most readily found; for the central volcanic island, rising from the 
lagoon within a barrier reef, was surely there while the reef was form- 
ing around it; and the features of the island shoreline will, as Darwin 
long ago pointed out for still-standing islands and as Dana a few years 
