178 



DALY. 



islands were all low and mostly composed of very weak materials. It 

 is probable that these gigantic embankments were only parth' veneered 

 with massive reef material; such comparatively strong rock was in 

 danger of undermining and rapid destruction as soon as the sea-level 

 fell 45 m. or more, because of glaciation in higher latitudes. 



On the other hand, the high volcanic islands were young. Their 

 rocks were still not essentially weakened by decay, and, as to-day, 

 largely consisted of strong, massive lava flows. Wave-benching in 

 such material would be incomparably slower than in that of the older 

 islands or of the continental shelves. 



Origin of the Coral-Recf Platforms. 



Size of the Actual Platforms. The lengths and breadths of the larger 

 reef platforms are typified in the following list : 



Ontong Java atoll (Solomon islands) 

 Macclesfield bank (China Sea) 

 Australian Great Barrier 

 Chagos Bank (Indian ocean) 

 Suvadiva atoll (Maldives) 

 Miladummadulu-Tiladummati atoll 

 (Maldives) 



145 



30 



The large majority of the platforms have lengths less than 30 km. 

 and widths less than 20 km. In a consideration of their possible 

 smoothing by surf action, width is nmch more important than length. 

 (See also Table I at page 187.) 



Pre-Glacial History of Volcanic Islaiids. Both a priori reasoning 

 and direct deduction from the knowai submarine contours about 

 islands like Hawaii and Tahiti, warrant the conclusion that the largest 

 and middle-sized platforms could not be caused by the abrasion of 

 young volcanic masses during the Pleistocene. To review the situa- 

 tion for the volcanic islands of great age, an ideal case may be 

 considered. Assume that a conical island of normal volcanic com- 

 position (mixed lava flows and ash beds) was formed so long ago as 

 to have been thoroughly planed down by erosion. The encircling 

 embankment of debris would contain the insoluble material washed 

 out of the island, and in this would be incorporated shells and skele- 



