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



those of New Caledonia, both of which I saw last summer, are 

 eloquent witnesses to the same conclusion, though they offer 

 their testimony so unobtrusively that it has received less atten- 

 tion than the more outspoken testimony of steep-sided embay- 

 ments. In both these cases, the cycle of erosion that preceded 

 the submergence by which the present coast line was embayed 

 and the up-growth of the present barrier reefs was permitted, 

 lasted long enough in certain districts to open mature valleys 

 in resistant crystalline rocks, and to reduce well indurated 

 stratified rocks to rolling lowlands scores of miles in extent ; 

 sometimes to local peneplains several miles in width. It seems 

 inadmissible to ascribe so great an erosional work to a time so 

 short as the sum of the glacial epochs. Hence it must be con- 

 cluded that the depression of the sea surface during the gla- 

 cial period was not great enough in amount or long enough in 

 duration to account for the pre-submergence forms of barrier- 

 reef islands. 



Several lines of evidence, which have not, I believe, been 

 considered before, thus lead to the rejection of changes of sea 

 level and of sea temperature, or of sea level alone, during the 

 glacial period as sufficient cause for the observable features 

 of the barrier-reefs and their associated islands in the archipel- 

 agoes that I visited : hence I must reject the processes of gla- 

 cial control as sufficient explanation of the atolls of the torrid 

 zone also. Changes of sea level during the glacial period 

 must have taken place in some undetermined measure, and 

 they may possibly have had secondary importance in permit- 

 ting the transection of formerly continuous reefs by stream-cut 

 notches which, now flooded, appear as "passes" not yet closed 

 by renewed coral growth ; or more probably in modifying the 

 rate of submergence and of reef-growth, as is briefly considered 

 in a later section (p. 267) ; but glacial control does not seem to 

 have been sufficient to produce the primary effects that have 

 been attributed to it. The effects of changes in sea level and 

 sea temperature during the glacial period seem to me to have 

 been subordinate to the larger effects of a more efficient cause. 



The Borders of the Coral Zone. — While the evidence given 

 above has led me to reject the glacial-control theory as afford- 

 ing an adequate cause for the features of the great majority of 

 coral reefs, it should be added that the equatorial belt of the 

 Pacific is not the best region in which to test a theory which 

 is based on the assumption that the reduction of ocean surface 

 temperature during the glacial period was sufficient to kill the 

 corals of the Pacific reefs. A better test of the theory might 

 be made on the borders of the coral zone, where the ocean is 

 now only just warm enough to permit the growth of corals, 

 and where the corals would have been first and longest 



