228 MAYOR— THE REEFS OF TUTUILA, SAMOA. 



such places, but the coral heads are small, and the spaces between 

 them usually wide. In places where the depth is as great as 8.5 

 fathoms, corals such as Acropora arcuata are not more than one 

 foot wide, whereas this species attains a diameter of three or four 

 feet in water not more than 2 to 4 fathoms in depth. 



These ancient reefs of Tutuila may have been drowned too sud- 

 denly to permit coral growth to maintain them at the surface, or 

 as seems more probable, conditions may for long periods of time 

 have been unfavorable for corals, thus permitting a gradual subsi- 

 dence or a slow rise of sea level to effectively drown the reefs under 

 a depth too great to permit the renewal of coral growth when con- 

 ditions became otherwise favorable. 



Moreover, we know from observation made at Tortugas. and 

 from those of Wood Jones at Cocos Keeling that a coral reef once 

 killed may not renew itself in half a century. At Tortugas the 

 Acropora muricata, w^hich constituted wide areas of shallow reef, 

 were killed by the " dark water" of October, 1878. and even to-day 

 (1920) it is a rare coral over the flats where once it was the domi- 

 nant species ; and very similar conditions are described by Wood 

 Jones in his " Coral and Atolls " for the lagoon of Cocos Keeling. 



Moreover, there are many places along the shore of Tutuila as 

 at Vatia, or near Fagaalu, where corals which once grew upon the 

 reef flat were torn loose and driven ashore by an unrecorded hurri- 

 cane which must have occurred more than fifty years ago, yet these 

 reefs have not recovered and are quite smooth and devoid of coral 

 heads. 



It appears that a reef once established can readily maintain itself, 

 but once it be destroyed many years may elapse before corals can 

 again attain a foothold. Thus on steep slopes if corals which die 

 or are broken ofif roll down into water too deep for coral growth, a 

 reef may be greatly hindered in establishing itself, and thus one may 

 have the condition seen in the Marquesas Islands where numerous 

 scattered coral heads are found growing upon the submarine vol- 

 canic slopes, but they have not yet succeeded in establishing reefs. 

 Coral reefs can readily form upon relatively flat submerged or sub- 

 sided platforms but steep slopes are unfavorable for their initiation 

 and maintenance. 



