THE OCEAN AND ITS WORK 247 



away had a combined area of at least 6,000,000 square miles, with an 

 average thickness of probably more than 3000 feet. The removal 

 of enough water to form that ice tended to lower sea level all around 

 the globe at least 150 feet. The gravitative attraction of the ice 

 caps must have further lowered the equatorial seas by amounts 

 ranging from 30 to 50 feet. The net shift of level in the equatorial 

 zone was, therefore, at least 180 feet. Conversely, the melting of 

 the full 6,000,000 square miles of ice must have raised sea level in 

 that zone about 180 feet." x The cooling of the climates and waters of 

 the world during the Glacial Period (p. 644) retarded, or entirely 

 stopped, the growth of reefs over a large part of the world. Having 

 lost their defending reefs by this temporary change in climate, the 

 islands were vigorously attacked by the powerful breakers of the 



Fig. 237. — Diagram illustrating the glacial-control theory of coral islands. 

 The platform rock is shown by dots, the coral reef and calcareous debris by solid black. 

 The level platform is thought to be a plain of marine denudation cut when the sea 

 level was lowered by the withdrawal of water to form the great ice sheets of Europe and 

 North America. The coral islands were slowly built up as the level of the sea was 

 raised upon the melting of the glaciers. (After Daly.) 



open sea, resulting in their planation at a depth of a few fathoms 

 below the level of the sea of that time (Fig. 237). With the ameliora- 

 tion of the climate, the ice caps of the high latitudes began to melt, 

 and the surface temperature of the equatorial ocean was soon raised 

 to a point which permitted the coral polyps to flourish. These 

 animals speedily colonized the eroded platforms and developed the 

 atoll form as the result of the slow rise of sea level. 



As proof of this theory it is pointed out that the platforms upon 

 which the reefs rest are remarkably flat, as if planed by marine erosion, 

 and that they have a nearly uniform depth of 275 feet, i.e., a depth 

 of about 95 feet below the level of the seas of glacial times. It will 

 be seen that Darwin's and the Penck-Daly theories differ principally 

 in that, in the former, the land is believed to have slowly sunk, 

 while in the latter, the sea level is thought to have been gradually 

 raised. 2 



1 Daly, R. A., — Pleistocene Glaciation and the Coral Reef Problem, Am. Jour. Sci., Vol. 30, 

 1910, pp. 297-308. 



2 Daly, R. A., — Science Cons pectus, Vol. 1, 1911, pp. 120-123. 



