Daly— The C oral-Reef Zone. 143 



The late-Pliocene reefs are thus considered to have 

 been almost wholly of the fringing type. The younger 

 fringes were narrow because of their youth. The older 

 fringes were probably broader, but their widening had 

 been kept comparatively slow, both by the narrowness of 

 many shelves and because of periodic smothering of reef 

 organisms by shelf sediment. 



Conditions During the Glacial Period. 



The physiographic influences exerted on the coral-reef 

 zone during every glacial and interglacial stage can not 

 be described in detail. From the facts and probabilities 

 in hand only most general deductions are possible. 



That some chilling of the coral seas took place with the 

 onset of each glaciation appears reasonable. A cooling 

 of the present sea by 5° C. would seriously affect the 

 vigor of reef growth. This point has been elaborated in 

 the 1915 paper. Setchell, the leading expert on the 

 temperature relations of the marine algae, holds that the 

 tropical algae, including the large genus Lithothamnion, 

 are highly steno thermic, so that a general chilling of 5° 

 in the tropical ocean might adversely affect the growth 

 or reproduction of Lithothamnion. 10 Since Lithotham- 

 nia are now reputed to be at least as important as corals 



acceptable definition of " coral reef" can not now be framed. However, 

 the structures referred to under that name by Darwin and later students of 

 the reef problem, have typically two features in common: an essential pro- 

 portion of reef corals in each ridge-like mass, and competence of the struc- 

 ture to resist wave abrasion. Mere abundance of corals is not of itself a 

 sufficient criterion, as Darwin long ago stated (Coral Beefs, chap. 4, section 

 1). On the other hand, ability to withstand the breakers is absolutely nec- 

 essary to the typical, sea-level reef. In order, then, to prove that an 

 uplifted coralliferous limestone represents an ancient reef in the Darwinian 

 sense, the former presence of a defensive rim of growing corals and other 

 cooperative organisms must be demonstrated. On account of erosion, here 

 relatively speedy, proof of a former rim may be difficult or impossible. 

 Hence the observer must often remain in doubt as to whether he is dealing 

 with a calcareous sea-level deposit formerly guarded by thriving corals or 

 with a reefless bank, shallow enough to permit the growth of reef corals, 

 which, however, were nowhere assembled after the fashion of a true reef. 



Lacking the protective rampart, many shores may thus have undergone 

 prolonged abrasion by waves, although on the shelves offshore, reef corals 

 were growing. On the flat floor of a sinking geosynclinal, coralliferous 

 limestones may accumulate to indefinite thickness without the formation of 

 any reef in the ordinary meaning of that term. A coral patch is not a reef 

 in Darwin's sense; isolated corals or even lenses of corals in elevated lime- 

 stone are not proofs of former reefs. Failure to observe this has led to some 

 serious errors in reasoning and argument. 



10 W. A. Setchell, personal communication, and Annals, Missouri Botanical 

 Garden, vol. 2, pp. 287-305, 1915. 



