;>(I4 Ihtlij — Plcitnlocane Glaolaiion ainl Coi'dl licef Prohlem. 



wiutiM- toniperature below 20° C. Tlic Kebriiai-y marine 

 isothenii of 20° lies but a few limuli-ed miles north of tlie 

 larorr Hawaiian islands. We may be certain that those 

 islands were not bordered by livin<>- reefs at a time when 

 -I-,000,000 square miles of ice capped the neii>-hborin<y continent. 

 Hawaii itself seems to have borne at least one small glacier, 

 the characteristic traces of which were observed by the writer 

 on Mauna Kea at the 12,000-foot level. The supposition that 

 the present Hawaiian reefs be<>;an their growth in the ])ost- 

 Glacial interval is corroborated by the fact of theii' small size ; 

 they have evidently been growing but a short time. 



The latter statement may also be made concerning the 

 majoi'ity of the coral reefs of the globe. 



Is it not fair to believe that the reef corals were coniined, 

 during the Glacial period, to very limited areas, where pit)- 

 longed winter cold did not surpass 20° C. ? The reef corals 

 were, of coui-se, not exterminated during the Pleistocene, but 

 they may well have been resti'icted to such warm, more or less 

 enclosed seas as those of the East Indiau region. 



The imagination suffers, therefore, no painful stretch in our 

 considering that, throughout the Glacial period, extensive 

 marine abrasion in the equatorial belt was possible because of 

 a geueral lack of growing coral reefs in the open ocean. The 

 undefended islands, largely composed of relatively weak vol- 

 canic and calcareous materials, must, under the conditions, 

 yield extensively to the waves, which as a rule ran in from 

 very deep water on every side and thus, with sj)ecial power, 

 attacked the islands. 



All com])etent observers agree that most of the atoll-forming 

 reefs are founded on denuded volcanoes. Judging by the 

 analogies found in Plawaii, Samoa, Fiji, etc., a large propoi'tion 

 of these truncated volcanoes, in their subaerial portions, were 

 composed of massive lavas as well as of very much weaker ash 

 and tuff deposits. To reduce such masses, wave erosion would 

 demand vastly more time than is represented in the Glacial 

 period. But there is no reason to doubt that the volcanoes 

 here considered are of many different ages, possibly from the 

 pre-Cambrian to the Tertiary. For the older ones, subaerial 

 denudation must have gone still further than it has in greatly 

 dissected islands, like Kauai, Tahiti, etc.. and tlius approached 

 or reached peneplanation of the islands. Such denudation, 

 combined with marine erosion during pre-Pleistocene time, 

 had reduced most of the volcanic masses to the plateau form. 

 As the recently elevated islands so often show, many of these 

 platforms became veneered with, and flanked by. Tertiary 

 limestone, and doubtless they were often capped by coral reefs 

 of much greater total area than that of the existing coral 

 reefs. 



