Daly — Pleistocene Glaciation and Coral Reef Problem. 303 



the depth of the enclosed lagoon has not yet been completely 

 explained. Thus, the view that the crown has been built 

 lip by corals during positive movement has still some foun- 

 dation."* 



That the plateaus are due to erosion, coupled with the depo- 

 sition of the eroded material as an advancing submarine talus, 

 seems to be the preferable explanation. A full discussion of 

 the point will not be made here, as it would demand many 

 pages compiling the arguments of the older hypotheses and 

 would thus unduly lengthen the present note. Suffice it to 

 say, that in crediting the forms and hypsometry of the plateau 

 surfaces essentially to marine abrasion, we make the only 

 assumption which seems able to match all the facts. 



It is not likely that the plateau surfaces were shaped when sea- 

 level had its existing relation to the general sea bottom. Even 

 the Pacific breakers cannot effectively erode the bed-rock of a 

 broad shelf with depths approaching 45 fathoms. It is hard 

 to believe that they could lower the plateau surfaces to such 

 depth in any permissible time. 



On the other hand, the conditions for the development of 

 the platforms by marine abrasion were met during the Glacial 

 period. For a large part of that epoch the level of the inter- 

 tropical ocean was about 30 fathoms lower than now. Cham- 

 berlin and Salisbury have estimated that the length of the Glacial 

 period was from 300,000 years to more than 1,000,000 years.f 

 The entire time during which there was heavy glaciation, 

 comparable to that of the maximum ice-caps, must have been 

 much more than 100,000 years. The Pleistocene islands 

 were subjected to the attack of waves and currents operating 

 near a sea-level which shifted up and down with the maxima 

 and minima of glaciation ; but we may conceive that the net 

 result was the preparation of broad benches and platforms at 

 depths ranging from zero to fifteen or twenty fathoms below 

 the last of the low Pleistocene sea-levels. 



In a second principal way the Pleistocene conditions of 

 abrasion in the equatorial ocean probably differed from those 

 now ruling in the same latitudes. Some of the best authorities 

 on the Glacial period hold that the whole surface of the earth 

 was chilled during that time.:}: A fall of only half a dozen 

 degrees in the average temperature of the intertropical 

 seas would make an enormous difference in the distribution 

 of the stenothermic i-eef-corals, which cannot withstand a 



* E. Suess, The Face of the Earth, trans, by H. B. C. SoUas, vol. iv, 1909. 

 p. 327. 



tT. C. Chamberlin and R. D. Salisbury, Geology, vol. iii, 1906, p. 420. 



X H. Hess, Die Gletscher, Braunschweig, 1904, p. 398 ; T. C. Chamberlin 

 and R. D. Salisbury, Geology, vol. iii, p. 327, 1906 ; A. Penck, Morphol- 

 ogie der Erdoberflaeche, vol. ii, pp. 528, 660, 1894. 



