226 MAYOR— THE REEFS OF TUTUILA, SAMOA. 



ancient harbors although such would have been the case were coral 

 reefs growing along the shores at the time when this emerged bench 

 was at sea level. Moreover, in places as at Fagatoga this old shore 

 bench was partially covered by talus sHding down from the cliffs, 

 but upon digging this talus away no elevated corals or limestones 

 were found. We are thus forced to conclude that during the time 

 when the sea was at its highest level and cut the now emerged 

 shore-bench, there were no coral reefs growing around Tutuila. 



The fringing reef which is now growing outward from the 

 sihores around Tutuila, in places where the slope is not too steep, 

 is of modern origin. It is 300 to 1,000 feet wide in places where the 

 underlying volcanic slope is gentle, and is narrow, or absent in 

 places where the submarine slopes are steep as ofif the seaward ends 

 of the basaltic promontories. Thus the reefs now growing out- 

 ward over the gentle, wave-worn slopes of the drowned valleys are 

 wider than those off the cliffed promontories at the mouths of the 

 harbors, and everywhere the width of the reef is a factor of the 

 steepness of its underlying volcanic substratum. 



The fact that narrow reefs are found on steep submarine slopes 

 and wide ones on gentle slopes was pointed out iby Darwin in " Nat- 

 uralist's Voyage around the World," 1873, p. 472; also A. Agassiz, 

 1903, Proc. Royal Soc, Vol. 71, p. 414, says that the absence of 

 coral reefs in the Marquesas Islands is due to the steepness of their 

 slopes. 



Tutuila is very ancient according to Daly, who has made a 

 survey of its lithology (Year Book of the Carnegie Institution No. 

 18, 1919). Indeed it shows clear evidence of considerable subsi- 

 dence, the harbors of its northern coast such as Fagasa, Vatia, 

 Afono, Massefau and Aoa being typical drowned valleys, while 

 Pago Pago Harbor, according to Daly, shows characteristics of a 

 drowned valley but has had a complex history. 



W. M. Davis (1918, Bulletin U. S. Geological Survey, p. 523) 

 observed that the data for the unpublished Hydrographic Ofifice 

 Chart of Tutuila " reveals the existence of a submerged platform 

 from one to three miles in width and from 30 to 50 or more fathoms 

 in depth," and also that " the outer part of the platform is usually 

 somewhat shallower than at half distance ofif shore as if a poorly 

 developed barrier reef enclosed it." 



