240 W. M. Davis — Shaler Memorial Study of Coral Reefs. 



largely composed of relatively weak volcanic and calcareous 

 materials, must . . . yield extensively to the waves, which as 

 a rule ran in from very deep water on every side and thus, 

 with special power, attacked the islands " (304) ; he adds, how- 

 ever : " The lava-formed islands of late Tertiary or Pleistocene 

 dates . . . would stoutly resist abrasion '' (305). But cliff- 

 rimmed central islands are not known, with the exception 

 of New Caledonia and Tahiti, as stated above, and Tahiti 



Fig. 5. 



Fig. 5. 



theory. 



Deduced stages of a barrier reef, according to the glacial-control 



is a " lava-formed island," which Daly places with those that 

 should " stoutly resist abrasion." [No account is here taken 

 of certain islands in which two or three spurs are cut off in 

 cliffs, while twenty or thirty are not cut off : these will be con- 

 sidered in my detailed report.] Hence it appears impossible 

 to explain both wide and narrow lagoons by the theory here 

 under consideration. Escape from this dilemma may be found 

 by assuming that the corals of narrow-lagoon barrier reefs were 

 not killed by lowered ocean temperatures so soon as those of 

 wide-lagoon atolls : but this assumption is too arbitrary to be 

 acceptable. Hence, as far as these two lines of evidence go, it 

 must be concluded that the corals were generally not killed 

 during the glacial period, and that lagoon floors are generally 

 not abraded platforms. 



The Valleys of Barrier-reef Islands. — Furthermore, if the 

 embayments of a central island within a barrier reef result 

 from the drowning of valleys that were eroded with respect to 

 the lowered sea level, B, fig. 5, of a relatively short glacial 

 period, then each such valley must be entrenched in the floor of 

 a preglacial valley ; and above the head of each embayment 

 resulting from the drowning of a new-cut valley, there should 

 be a " valley-in-valley " landscape, as in sector C, unless the 

 preglacial valley was so young and narrow that its sides were 

 undercut and destroyed by the deepening and widening of the 



