158 Daly — The Coral-Reef Zone. 



should not obscure the validity of the more general 

 theory. 



Demonstration of the general process must wait on a 

 double course of study — observation of sea, land, and 

 organisms today, and reasoned inference regarding the 

 condition of sea, land, and organisms at the successive 

 stages of oceanic evolution. This paper specially 

 stresses the latter, historical or geological, mode of 

 attack. Its leading conclusions may be noted. 



1. At the present time the tropical ocean is probably 

 characterized by an exceptional abundance of vigorous 

 coral reefs. In pre-Glacial time, atolls and barrier reefs 

 were much rarer, if not entirely absent from the sea. 

 Pre-Glacial fringing reefs were more in danger of 

 smothering by sediment than are most existing reefs. 

 Hence the Pliocene and older open-ocean islands may not 

 have been nearly so well defended against marine abra- 

 sion (ultimate truncation) as the existing oceanic islands. 



2. During Pleistocene glaciation, the prosperity, if not 

 the life, of reef corals was threatened by two enemies: 

 oceanic chilling and smothering by sediment. Perhaps 

 killing by wave-stirred mud and sand was more impor- 

 tant than the fall of oceanic temperature. The question 

 is raised as to whether the Lithothamnia, so vital to reef 

 strength, were also affected by these physical conditions 

 in the Pleistocene ocean. 



3. Working at levels lower than the present level, the 

 ocean waves of the glacial stages benched the weak strata 

 of the new coastal plains, the somewhat more resistant 

 pre-Glacial reefs, and, in far less degree, the harder shore 

 rocks. Tidal and other currents cooperated with the 

 waves in smoothing the outer banks. The actual amount 

 of cliffing now to be observed on tropical shores has been 

 once more explained in order to clear up a serious mis- 

 understanding of the Glacial-control theory. For a sim- 

 ilar reason, the history of drowned valleys in islands and 

 continents is again reviewed. While some drowned val- 

 leys betoken subsidence, their existence does not seem to 

 be crucial in the problem of coral reefs. 



A specially notable effect of the lowering of sea-level 

 during glaciation is seen to have been the shallowing 

 of water on the banks and shelves then existing, so that 

 corals could take root on the edges of these submarine 

 plateaus. In comparison, the platforms due primarily to 

 Pleistocene abrasion are less important. 



