220 DALY. 



A favorably placed fringing reef may have a width greater than that 

 of a typical annular reef, but that cannot be true of shore reefs which 

 are much troubled by invasions of mud. The observed widths of 

 fringing reefs, including their maximum values, also agree with the 

 implications of the Glacial-control theory. Therein the writer's 

 initial problem — an explanation of the youth of the HaM^aiian reefs — 

 finds a solution. It is worthy of note that Darwin's recognition of the 

 youthful appearance of atolls and other reefs, in both the Pacific and 

 Indian oceans, was a leading reason for his invention of the subsidence 

 hypothesis. ^-^ 



Objections to the Glacial-control Theory. 



The preferred explanation of coral reefs contains many elements 

 involving definite quantities. These include: the actual temperature 

 of the Pleistocene ocean; the volumes of water abstracted from the 

 sea during the various maxima of glaciation; the effect of glacial 

 attraction on tropical sea-level ; the length of the whole Glacial period ; 

 the total duration of the greater ice-sheets ; the power of Pleistocene 

 waves; the rock-strengths and heights of the Pleistocene islands; 

 the length of time since the late-Glacial warming of the sea; the loca- 

 tion and amounts of post-Glacial (Recent) deformation of the earth's 

 crust underlying the coral seas; the growth rates of corals and of their 

 reef -building allies; and the maximum and mean depths of reef 

 lagoons, island shelves, and continental shelves. In a recent paper 

 Davis has expressed doubts as to the validity of the new theory, on 

 various grounds. Some of his reasons have to do with the quantita- 

 tive, elements just noted, and may be first noted, along with related 

 objections implied in other writings. ^^ Some repetition of statement 

 is advisable, in the interests of clearness. 



Glacial Lowering of Sea-level within the Tropics. "The maximum 

 number of feet by which the sea-level was lowered may have been less 

 than the amount above quoted (200 feet), because the depression of 

 certain glaciated lands, like Labrador and Scandinavia, while ice 

 sheets lay on them, was presumably compensated by an uplift of the 

 neighboring sea floor, and that would have tended to raise the sea- 

 level." (Davis, p. 729.) The depression of these lands, if no greater 

 than their post-Glacial uplift, would not aflect the order of magnitude 



51 C. Darwin, Coral Reefs, London, 3rd ed., pp. 43 and 69 (1889). 



52 W. M. Davis, Bull. Amer. Geog. Soc, 46, 728-734 (1914). 



