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Transactions. 



its formation (Davis, 1916c). The nearly horizontal interior strata, L, 

 may contain coarse sand near the reef-wall, fine lagoon deposits in the 

 middle, and volcanic sands and gravels near the central island ; and the 

 Avhole mass, with the exception of some of the outer slanting layers, may 

 lie unconformably on a rock surface of subaerial erosion. In such a case 

 reef-upgrowth during prolonged submergence probably due to subsidence 

 would be inferred. Irregularities in the reef-wall, as in fig. 9, would 

 indicate changes in the rate of submergence. A horizontal outgrowth, H, 



Fig. 9. 



would occur during a long still-stand . period, when delta ])lains, E, might 

 almost fill the lagoon. The occurrence of a buried cliff and platform in 

 the profile of the underlying rock, and an exterior detrital deposit, as shown 

 in figs. 2, 4, 5, 8, and 9, would be of much theoretical interest. 



In case dissected atolls are found, their structures should be studied 

 with especial care ; and if their rock foundation is disclosed it should be 

 closely examined to see whether the atoll limestones lie on it conformably 

 or not. Christmas Island, in the eastern Indian Ocean, merits renewed 

 study in this respect, for basalt has been seen in ravines behind its lime- 

 stones, but the nature of the limestone-basalt contact has not been fully 

 described. 



Ficr. 10. 



In case elevated reefs occur in terrace-like arrangement, one above the 

 other, as on Cebri in the Philippines, and elsewhere, the structure of the 

 successive terraces will indicate the sequence of formation of their reefs. 



