188 CARNEGIE INSTITUTION OF WASHINGTON. 



wide as many of the ancient extinct barrier reefs. Thus they appear 

 to be of relatively recent formation and all of about the same age, and 

 this fact lends support to Daly's idea that the living reefs of the Pacific 

 have grown upward upon submerged platforms since the end of the 

 last glacial period. 



In one respect, however, Tutuila seems to lend little support to 

 Daly's glacial-control theory, for when the sea stood about 180 feet 

 lower than at present there were wide fringing reefs and barrier reefs 

 around the island, and thus the clunate was apparently tropical 

 while according to the theory the glacial period was then at its height. 



Moreover, Daly and Chamberlin were unable to demonstrate that 

 when the sea stood highest, coral reefs grew around the island, and 

 thus if present were probably not extensive. 



In Samoa corals were planted in concrete and grown at various 

 depths from 50 feet to near the surface. The maximum growth-rate 

 appears to be in places washed by pure agitated ocean water of moder- 

 ate depth, as on the seaward slopes of the reefs. Here about three 

 quarters of the surface is densely covered by large, thick-stemmed 

 heads of Acropora and Pocillopora, these being the coral genera which 

 grow most rapidly. Our observations in Samoa show that a reef com- 

 posed of the slow-growing massive Pontes, such as live in impure 

 water and in silted regions, might grow upward 120 feet in 2,000 years, 

 but Acropora grows at about four times this rate. Thus the modern 

 fringing reefs of Samoa could readily have been formed since the close 

 of the last glacial period, and may not be more than 30,000 years old. 



The fast-growing Acropora leptocyathus and Acropora arcuata cluster 

 thickly along the breaker-washed overhanging edge of the reef, and 

 when they die they are largely preserved in place by being covered 

 with lithothamnion. This lithothamnion veneers all the dead corals 

 and smoothes the surface, forming a ridge about 6 to 8 inches above 

 low-tide level up which the breakers glide, constantly losing force, so 

 that they can not destroy the dense clusters of delicately branched 

 Acropora which grow in the shallow water of the reef-flat just back of 

 the crest of the lithothamnion ridge. 



Below the overhanging seaward edge of the reef there is a sheer 

 precipice usually from 2 to 9 fathoms in depth. At the foot and over 

 the steep sides of this precipice one finds a dense growth of Acropora 

 and Pocillopora, and these corals constitute the rugged floor at the 

 base of the precipice. The surges drive to and fro over this uneven 

 floor, breaking off slender tips of corals, so that only tough-stemmed 

 forms can survive in this agitated region, which is constantly worked 

 over by the strong currents driven by the breakers. Broken corals 

 are jammed tightly into crevices, and dead ones are covered by layer 

 after layer of lithothamnion, and thus a compact wall is built upward, 

 while the overhanging lithothamnion-protected, breaker-washed sea- 

 ward edge of the reef advances over it and covers it with its talus. 



