218 DALY. 



late Pleistocene. That the depths are relatively great as compared 

 with most lagoon depths, is at once largely explained by the failure of 

 continuous reef growth, on which the post-Glacial aggradation of the 

 platforms has so much depended. 



Among the Solomon islands, Guppy found that the reefs either 

 reached the surface or had 7 m. to 18 m. of water upon them. Reefs 

 of intermediate depth were not found. ^^ His generalization suggests 

 the necessity of a rather definite critical strength which must be sur- 

 passed if a reef is to keep its summit at sea-level. If its power of 

 resistance to the waves falls below that critical point, the top of the 

 reef is kept down to levels ( — 7 to — 25 m.), where the abrading surf can 

 no longer conquer in the struggle with living coral and its allies. The 

 exact causes for varying success in the ceaseless combat are here again 

 not determined, and evidently mud-control is only one of many 

 factors, which are quantitatively varying from place to place in the 

 oceanic area. 



Nevertheless, the clear possibility that mud-control really explains 

 the Great Chagos -and similar "drowned" atolls, seriously affects 

 what Dana described as " one of the best demonstrations of the sub- 

 sidence theory." 



Volumes of the Existing Reefs. According to the new theory, the 

 living coral reefs rest on platforms prepared in the Glacial period, and 

 thus, in general, rest on pre-Glacial sediments or volcanic rocks. The 

 greatest thickness possible for these reefs is about 110 m., assuming an 

 extreme amount, 75 m., for the rise of post-Glacial sea-level within the 

 tropics. Usually the thickness would be less. 



Once more the theory can be tested quantitatively. The boring 

 at Funafuti showed massive coral to persist to a depth of about 46 m. 

 Below that depth the log of the boring suggests that it passed through 

 talus material all the way to the bottom, at a depth of 340 m. This 

 conclusion was reached by the writer after a careful study of the 

 Funafuti report, issued by the Royal Society of London; a subsequent 

 inspection of a duplicate set of the core material has tended to confirm 

 the opinion. Unfortunately, the hole bored in the lagoon at Funafuti 

 was not deep enough to decide the nature of the rock beneath the 

 lagoon detritus. As shown in a later section (page 247), the main bore 

 at F'unafuti, useful as it has been in clearing up many important points, 

 was badly located for its primary purpose of testing the Darwin-Dana 

 theory. A truly valuable test can be made by boring on a coral islet, 



49 H. B. Guppy, Proe. Roy. Soc. Edinburgh, 13, 867 (ISSti). 



