208 W. M. DAVIS 
residual stack surmounting a platform of marine abrasion; it is 
of dense volcanic rock, hence it probably represents a well-denuded 
volcanic summit on which the surrounding lagoon limestones rest 
unconformably. If the buried volcanic slope be such as prevails 
in the dissected islands of the Fiji group, the thickness of the 
Maré reef at the margin must be 5,000 feet or more. The volume 
of limestone thus accumulated is truly formidable; but if the 
theory of subsidence be proved correct on other grounds, we can 
hardly object to it because it involves large quantities. 
Reef E is a true atoll, like Funafuti, in which no volcanic central 
knob is to be seen. The upgrowing reef is here shown as slanting 
more and more inward, the longer its exterior talus becomes, for 
reasons that I have elsewhere set forth.t So long as an atoll 
remains at sea-level it is impossible to determine whether its 
structure accords with the demands of Darwin’s theory or of any 
other theory. Penetration of its structure by boring is difficult; — 
the boring at Funafuti, 1,114 feet deep, was made on the marginal 
reef; it is shown in true proportion to the diameter of the atoll by 
the vertical lme at E:* a boring near the lagoon center would, 
according to the subsidence theory, have been much more likely 
to reach a volcanic foundation; and such a boring at Bermuda 
reached volcanic rock at a depth of 245 feet below sea-level and 
penetrated it for 1,033 feet farther: several such borings would be 
needed to demonstrate the form of the buried volcanic mass, as is 
further noted below. 
If a long stationary period should supervene in the history of 
an atoll, its lagoon would be gradually filled and converted into 
what may be called an atoll plain; if such an atoll plain should sud- 
denly subside, narrow young reefs would grow up from its surface 
in such a manner as to suggest the independent origin of the two 
forms. Im so far as the Maldives offer examples of this kind their 
™“Extinguished and Resurgent Coral Reefs,’ Proc. Nat. Acad. Sci., II (1916), 
pp. 466-71. : 
2 As drawn in Fig. 1, the boring passes from the true reef into the lagoon deposits, 
because the upgrowth of the reef is here inclined inward; if the boring had penetrated 
such a reef as C, it would have remained for its whole depth in the reef proper; if it 
had penetrated a horizontal or outslanting reef, like the lower part of reef C, it 
would have passed through the reef into the underlying talus deposits. 
