W. M. Davis — Shaler Memorial Study of Coral Reefs. 225 



surf on its exposed front, swept over by the foaming surge, 

 and backed by the quieter waters of the lagoon. As far 

 as the facts there visible are concerned, a coral reef may 

 be the result of upward or outward growth from a still- 

 standing foundation, as Murray supposed ; it may be merely a 

 veneer on the outer edge of a wave-cut platform, as Agassiz 

 thought ; or an upward growth from submarine lava-flows, as 

 Guppy suggested ; it may have been formed by upward growth 

 during the rise of sea level as the continental ice-sheets of the 

 glacial period melted, the upward growth having been begun 

 on the outer edge of a platform down to which preglacial reefs 

 were abraded by wave action while the sea level was lowered 

 by the withdrawal of water to form continental ice-sheets, and 

 while the corals of the reefs were killed by the lowered 

 ocean temperatures of the glacial period, as Daly has recently 

 argued ; it may have been formed by upward growth around a 

 slowly subsiding island, as Darwin long ago believed. Sea- 

 level coral reefs, taken alone, do not afford any sufficient test 

 by which the true theory — that is, the correct mental counter- 

 part of the unobservable facts of the past — can be detected. 

 Hence appeal must be made from the non-committal sea-level 

 reefs to competent witnesses of some other kind, which were 

 present while the reefs were forming and which are willing 

 to testify about the events which then took place. The testi- 

 mony of such witnesses will be presented in terms of the 

 changes which they themselves suffered during the growth of 

 the reefs; and these changes, if still recognizable, ought to 

 supply evidence as to the conditions and processes under which 

 the near-by reefs had their origin. The evidence thus secured 

 should contradict all the erroneous theories and confirm the 

 correct one. 



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, fig. 1, are concerned, their origin is 

 hardly in debate : they are growing colonies of corals, initiated 

 by the arrival from elsewhere of passively floating larvae 

 which establish themselves in shallow water close around a 

 newly offered and suitable reef-free shore line under fitting 

 conditions of temperature ; the reef thus formed being after- 

 wards enlarged by upward or outward growth without other 

 important change ; second, that as far as atolls — or reef -rings 

 enclosing shallow lagoons — are concerned, they are inscrutable 

 unless penetrated by numerous and expensive borings, 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 foundations on which they were formed will give 

 important evidence regarding their origin, and should therefore 



