THE REEFS OF TUTUILA, SAMOA, IN THEIR RELATION 

 TO CORAL REEF THEORIES. 



By ALFRED GOLDSBOROUGH MAYOR. 

 (Read April 22, 1920.) 



The Island of Tutuila, American Samoa, is purely volcanic, no 

 elevated limestones having been observed upon it. As has been 

 shown by Daly the exposed volcanic rock is in most places weath- 

 ered to its ulttimate degree, but this does not apply to the region of 

 the southwestern shore between Tafuna and Sail Rock, where a 

 lava flow occurred in times so recent that its surface still shows a 

 ropy structure, while sharp-edged cracks and numerous lava cav- 

 erns remain open quite as they were at the time of their formation. 



The Island is strongly cliffed by the sea, some of these clififs as 

 at Round Bluff being fully 300 feet high. After forming these 

 clififs the sea level sank so as to expose a platform of hard volcanic 

 rock which projects from 50 to 250 feet seaward from the foot of 

 the basaltic promontories. The recent lava flow between Tafuna 

 and Sail Rock overwhelmed this platform but elsewhere practically 

 every promontory of the Island shows this old emerged shore-shelf, 

 and remnants of it appear in numerous places lying ofif shore within 

 the bays, such as in Afonu, Vatia, Fagasa, Leone and Pago Pago 

 harbors, and especially along the south coast between Breaker Point 

 and Fagaitua Bay. It is also found around the ofif-lying island of 

 Aunuu southeast of Tutuila. 



The bays of Tutuila are in regions of fragmentary material and 

 are flanked by ridges of harder rock. Thus the emerged shore 

 platform has resisted ero&ion where it is composed of basalt as at 

 the spur ends, but has largely disappeared within the harbors where 

 the rocks are generally soft. 



A similarly elevated shore-shelf of volcanic rock is found around 

 the islands of the Manua Group, Ofu, Oloosega and Tau, and also 

 at Rose Atoll where it is of hard dolomitized limestone bearing 



224 



