GLACIAL-CONTROL THEORY OF CORAL REEFS. 219 



situated within the lagoon of a typical atoll, about midway between the 

 main reef and the lagoon center. Among the more favorable locations 

 is that at Breakfast (Friihstuck) Island in the Jaluit atoll. 



A second test is to be found in the thicknesses of open-ocean reefs 

 which have been uplifted in the post-Glacial period. By the new 

 theory they should never surpass 75 m. to 110 m. If the uplift has 

 been great enough, the unconformity of reef and basement should be 

 visible. After a fairly complete review of coral-reef literature, the 

 writer is convinced that these theoretical consequences accord well 

 with the facts so far published. That the existing reefs are mere 

 veneers is the accordant testimony of Agassiz, Wharton, Semper, 

 Gardiner, Guppy, and others of those who have studied recently 

 elevated islands. Speaking of the Solomon group, Guppy wrote: 

 " Amongst the numerous islands that I examined, I never found one 

 that exhibited a greater thickness of coral limestone [i. e., true coral- 

 reef material in situ] than 150 feet [46 m.], or at the very outside 200 

 feet [61 m.j." ^° His statement in principle serves to express the view 

 held by each of these experienced observers. 



Again, the Glacial-control theory implies that the areal extent of 

 the existing reefs must be small. Post-Glacial time is now proved to 

 be quite moderate, of the order of 20,000 to 50,000 years. Yet, if a 

 period of 100,000 years measures more closely the life of many existing 

 reefs, these still cannot cover more than very small portions of their 

 respective platforms. As a matter of fact the dry land of the average 

 great atoll totals less than one per cent of the platform area. The 

 whole reef crown of such a typical atoll as Suvadiva totals only about 

 10 per cent of the platform area. Such proportions illustrate the 

 youth of the existing reefs. 



The lateral spreading of each reef is chiefly effected by growth of the 

 corals at depths of 9 m. to 35 m., the increase being, however, much the 

 more rapid on the open-sea side. If Gardiner's estimates of the rate 

 of coral growth apply to the whole Recent period, the observed widths 

 of the reefs would appear to demand from 20,000 to 50,000 years for 

 their development. In other words, the widths as well as heights of 

 the existing barrier and atoll reefs are of the proper size, if these cal- 

 careous rims originated on the platforms in post-Glacial time. (See 

 Figs. 5-22, 38-43.) 



Originating in very shallow water, reefs of the fringing class must 

 have smaller average thicknesses than either barrier or atoll reefs. 



50 H. B. Guppy, The Solomon Islands, London, p. 71 (1887). 



