Daly — Pleistocene Glaciation and Coral Reef Prohletn. 305 



The Pleistocene islands, emerginty because of the negative 

 inovoDient of sea-lovel, were, therefore, generally low (under 

 300 feet* in height), and, above the new sea-level, were com- 

 posed lai-gcly of weak material. Exposed on all sides to 

 abrasion by the open ocean, we may safely assume that these 

 islands would lose substance at least as fast as the 300-foot 

 chalk cliffs of Dover, namely, at the average rate of 3 feet 6 

 inches a year. At that rate, the Chagos Bank, by far the 

 most extensive of the plateaus here considered (sixty miles by 

 ninety miles), could have been reduced to the Pleistocene 

 sea-level in about 50,000 years. The whole duration of nearly 

 maximum glaciation in the Pleistocene was, as we have seen, 

 much longer than that. The estimate allows nothing for 

 that fraction of the Chagos plateau which is due to off-shore 

 deposition during the abrasion ; but this process, complementary 

 to the abrasion, obviously explains a notable part of each 

 plateau area. 



We may conclude that marine erosion, under Pleistocene 

 conditions, was amply competent to reduce most of the Ter- 

 tiary thalassic islands to a low Pleistocene sea-level (see cols. 

 3, 4, and 5 of table). The same is true of the Australian 

 continental shelf and of the similar shelves of New Guinea, 

 Fiji, etc. Though the Australian shelf reaches 100 miles in 

 width and was exposed to erosion on only one side, a large 

 part of it must have been composed of weak sediments; these 

 were rapidly benched by the Pleistocene breakers. 



On the other hand, the lava-formed islands of late-Tertiary or 

 Pleistocene dates, like Hawaii, Tahiti, Savaii, Murea, Bora Bora, 

 and Huaheine, would stoutly resist abrasion. Their benches 

 are, accordingly, not more than about two miles in width. 



Growth of the exintiny Reefs. — The melting of the huge 

 ice-caps must have been a slow process. They began to dis- 

 sipate, doubtless through a general amelioration of climate. 

 We may believe that the new platforms were colonized by 

 reef-building species quite early in the melting stage and 

 before the rising sea-level meant water of more than twenty 

 fathoms, that is, water too deep for most reef-building species. 

 Though we do not know precisely the average rate of growth 

 on coral reefs, we do know that it is, geologically speaking, 

 very rapid. There would seem to be no difficulty in assum- 

 ing the rate as quite sufficient to keep the living zones of the 

 reefs well within the 20-fathom isobath of the deepening 

 ocean. The mechanism of growth, with the resulting develop- 

 ment of atoll, bari'ier, and fringing reef forms, would be 

 analogous to that imagined by Darwin and Dana on the sub- 



* This figure chosen on the assumption that the Antarctic and Greenland 

 ice-caps were, perhaps, first formed in Quatei'nary time. 



