158 DALY. 



Outline of Theory. 



A FIELD study in the year 1909 impressed the writer with the nar- 

 rowness of the coral reefs about the Hawaiian islands. In A'ievv of the 

 proved rapidity of coral growth, this narrowness suggested that the 

 reefs are geologically very young. ■"• The discovery that a considerable 

 glacier had left its traces on Mauna Kea, Hawaii, about 3,600 meters 

 above sea-level, directly indicated a possible connection between the 

 youthfulness of the reefs and the former climate of the archipelago. 



During the northern winter the surface temperature of the Hawaiian 

 shore waters is but little above the minimum at which reef corals can 

 thrive, namely, 20° Centigrade or GS° Fahrenheit. The northern 

 limit of possible reef growth in these longitudes is a line only about 

 800 kilometers north of Hawaii and the line is still nearer the islands 

 to the northwest. The mean annual temperature of coastal waters 

 about Vancouver island is 10° C. In the Glacial period that tem- 

 perature was nearly 0° C. The erratic boulders and striations on the 

 bedrock floor of the Mauna Kea glacier appear to have an antiquity 

 of the same order as that shown in the traces of Pleistocene sea-level 

 glaciers in Washington State and British Columbia. The conclusion 

 seems inevitable that corals could not thrive during the Glacial period 

 anywhere in the Hawaiian group. Hence, the existing reefs must 

 have been planted in the course of late Glacial or post-Glacial time, 

 thus explaining their youthfulness. 



The principle involved should ob\iously be tested by reference to 

 the facts known concerning the rest of the world's coral reefs. The 

 writer's efi'ort to do this led to a somewhat elaliorate hypothesis 

 covering the reef problem in general. After all of its essential ele- 

 ments had been recognized, the writer found that some of them had 

 already been described in published form. Yet no one had assembled 

 all necessary features of the explanation and a brief statement of the 

 whole, as then worked out, was published in 1910.^ The object of 



1 Throughout this paper the expression "coral reef" signifies the usual 

 complex of skeletal and shell growths, of which the frame-work is true coral 

 in situ, though a large part may be composed of nullipore or other algal mate- 

 rial, moUuscan or other debris of littoral species, shells of the plankton, and 

 chemically precipitated carbonates of calcium and magnesium. The corals 

 themselves may make up less than one-half of a reef, yet its existence and 

 increase depend on the successful growth of these animals in spite of a constant 

 battle with the surf. 



2 R. A. Daly, Amer. Jour, of Science, 30, 297-308 (1910); cf. Science Con- 

 spectus, pub. by the Massachusetts Institute of Technology, 1, 120-123 (1911). 



