176 P. A. Daly — P roblems" of the Pacific Islands. 



throughout the open ocean, the corals then beiug restricted to 

 the warmest border seas. 



Glacialists further assure us that the Glacial period was long, 

 with a total duration of hundreds of thousands of years. One 

 of the most trusted estimates is that by Chamberlin and Salis- 

 bury, which is close to 1,000,000 years. If, during a large 

 fraction of that long period, the ocean shores were not pro- 

 tected by living reefs, considerable wave-cutting must have 

 taken place. 



Pleistocene glaciation had a second chief consequence. The 

 present depth of the wave-cut benches evidently depends on 

 the position of the sea-surface in the Glacial period. As the 

 facts from Hawaii were further weighed, it became clear that 

 sea-level must have been generally lowered when the huge ice- 

 caps of the Pleistocene were formed. "Within the coral-reef 

 areas, sea-level must have been lowered also because of the 

 gravitative attraction of the ice-caps of higher latitudes. Each 

 of these deductions was found to be already anticipated by 

 several authors. The most probable estimate of the total 

 lowering for the tropical seas seems to be about 180 feet. 



The Glacial period was multiple. Several times its ice 

 sheets waxed greatly and waned greatly. Yet one may well 

 assume that, throughout most of the epoch, waves could suc- 

 cessfully attack islands which before had been protected by 

 reefs. For several hundreds of thousands of years the bench- 

 ing proceeded. During some tens of thousands of years, when 

 sea-level was about 180 feet lower than now, the open-ocean 

 surf could cut into the weaker shore materials, at a rate con- 

 servatively estimated as from three feet to thirty feet per 

 annum. When it is remembered that oceanic islands and 

 shoals are subject to wave action on all sides, there is no diffi- 

 culty in believing that many of the less resistant structures, 

 twenty miles or more in diameter, might be completely trun- 

 cated and smoothed by the waves, during merely the time of 

 maximum glaciation. Just as clearly the waves could make 

 little impression on hard lavas, so that young volcanic islands 

 should to-day have but narrow submarine benches. Narrow 

 or broad, the wave-formed facets would generally be from 

 thirty to ninety feet below the sea-level of the Glacial period 

 and from 200 to 300 feet below the present sea-level. Accord- 

 ing to the Glacial-control theory, it was on these platforms 

 that the existing, living reefs have grown. 



For clearness the ideal cross-sections of figs. 16 to 19 may be 

 reviewed. Section 16 is that of an ancient oceanic volcano. 

 Section 17 represents it after prolonged erosion has lowered 

 its surface, forming a detrital shelf (dotted) all around the island. 

 If not protected by coral reefs in a pre-Glacial period, the waves 



