MAYOR— THE REEFS OF TUTUILA, SAMOA. 



229 



A study of the submarine slopes of the fringing reefs of Tutuila 

 was made by casting out an anchor upon the seaward edge of the 

 reef, and then steaming seaward in the launch keeping the anchor- 

 line taut and making soundings at distances of 25. 50, 75, 100 feet, 

 etc., from the edge of the reef. This enabled us to determine the 

 slopes as shown in Fig. 3, which represents the conditions seen in 

 various parts of Pago Pago Harbor. The vertical and horizontal 

 scales are the same in these diagrams. We see that the growing 

 edge of the reef usually overhangs at sea level, due to the dense 

 clustering of the rapidly growing Acropora Icptocyathus in this 

 region, and to the fact that when they die these corals are main- 

 tained in place by the overgrowth of lithothamnion. Under this 



Fig. 3. Submarine slopes off the seaward edges of reef flats of Tutuila. 



overhanging edge there is a submarine precipice of usually 5 to 25 

 feet in depth, beyond which there is a narrow region which is rela- 

 tively flat, and where corals grow vigorously but cannot reach the 

 surface, due to the surges of the breakers which fracture their 

 stems in time of storm. Beyond this lies the seaward slope com- 

 posed chiefly of loose dead and alive coral fragments, the talus of 

 the reef, which extends downward at an average angle of about 25° 

 to 30°, thus being somewhat steeper than the average subaerial 

 slopes of volcanic islands. As this talus sets itself at an angle of 

 repose of 25° to 30° it would seem that a slope steeper than this 

 would be unfavorable for the starting of a coral reef, for whenever 

 a coral died or was broken ofif it would roll down the slope ; and new 



PROC. AMER. PHIL. SOC, VOL. LIX, O, JULY 22, I92O. 



