GLACIAL-CONTROL THEORY OF CORAL REEFS. 183 



the same depth below present sea-level. That depth depends: (a) 

 on the power of the waves, varying with storminess, length of fetch, 

 etc. ; (b) on the highly variable strength of the island formations ; (c) 

 on the original areas and heights of the islands ; and (d) on the differ- 

 ent values for the lowering of tropical-sea level because of gravitative 

 attraction by the ice. The control last mentioned is slight and for 

 general purposes may be neglected, though it may partly explain the 

 unusual depth of water now in the main Chagos lagoon of the Indian 

 ocean. Each of the other controls was doubtless of nearly equal 

 efficiency in the three oceans, so that in general the platforms should 

 be at depths ranging between 60 m. and 100 m. 



The zero depth for the facets is nearly illustrated in such a case as 

 Manga Reva (Gambier Islands), where small masses of hard volcanic 

 rock rise as islands in the midst of a barrier-reef platform, on which are 

 all depths down to 73 m. (40 fathoms). These islands are clearly 

 residuals of one or more larger volcanic masses. According to the 

 Glacial-control theory they have been separated by long subaerial and 

 marine erosion and not by crustal subsidence, as stated by Darwin. 

 Yet the wave-cut. Pleistocene facet does not appear at the present 

 shores of these and similar islands, partly because the latter were 

 subaerially eroded during the lower stand of sea-level in the Glacial 

 period ; some of the existing bays are drowned stream valleys. 



Depths of Lagoons and of Coastal Shclres in Stable Areas. The 

 attempt to state quantitatively the effect of Pleistocene wave abrasion 

 is seen to be laden with difficulties. However, a fair judgment seems 

 to indicate that composite benches, of area and depth corresponding 

 to the platforms from which the present coral reefs rise, were then 

 actually cut in islands and coastal plains. This conclusion is subject 

 to two different tests. 



Since their completion, the Pleistocene benches have been veneered 

 with shells and skeletons of pelagic organisms, with debris of coral 

 reefs, and with knolls and linear reefs of growing corals. Besides 

 sporadic, steep-sided knolls of coral, the lagoons, covering 95 to 99 

 per cent of the larger atoll platforms, have a nearly continuous bottom 

 layer of calcareous mud, sand, and organic remains. The smaller 

 the platform, the higher was the proportion of reef deljris in the \eneer, 

 and the more rapidly has the lagoon area been shallowed. (Figs. 

 5-11.) On the other hand, each large lagoon should be of nearly uni- 

 form depth over its central part. Wherever two or more large atolls 

 were subject to similar conditions for reef growth and for pelagic life, 

 the average depths of their lagoons should vary only within small 

 limits. 



