DEPARTMENT OF MARINE BIOLOGY. 189 



At depths greater tlmn 7 fathoms the coral heads become small and 

 appear to grow slowly ; and in this region the dead, loose talus of the reef 

 accumulates in jagged masses, the whole sloping downward at an 

 average inclination of about 30°, which appears to be the angle of repose 

 of the material. 



The lithothamnion which veneers the seaward edge of the reef does 

 not thrive except in agitated water, and it dies in the quiet region 

 back of the surges, and here the ridge breaks off in fragments to form 

 the loose, erratic blocks of dead coral veneered with lithothamnion 

 which the storm waves scatter over the floor of the reef-flat shoreward 

 of the lithothamnion ridge. Everything is loose in this region, whether 

 it be a coral head hundreds of pounds in weight or a mere grain of 

 corroded lithothamnion. Our studies show that these broken masses 

 of dead coral scattered over the reef-flat disappear at such a rate 

 that on the mid-region of the Aua reef, rocks of this sort weighing 

 5.1 pounds lose one-fifteenth of their weight in a year. The loss due 

 to this attrition and removal of sand by cun-ents is offset by the 

 growth of coral over the reef-flat, and thus as a whole it maintains 

 itself close to the level of low-tide, and is not disintegrated to form a 

 lagoon back of the growing seaward edge of the reef. Indeed, in the 

 Pacific, fringing reefs appear to be maintaining themselves as such 

 and are not changing into barrier reefs by shoreward solution of their 

 limestone, nor is there any e\'idence to support Darwin's view that atoll 

 rims were once barrier reefs. Apparently the living reefs of the Pacific 

 have arisen upon previously submerged platforms or ancient sub- 

 merged reefs, and these platforms have had a complex history which is 

 at present imperfectly understood. 



The history of Tutuila, according to Dr. Rollin T. Chamberlin, 

 appears to be as follows : A platform about 2 miles in width was cut 

 by marine erosion around the island. Then this platform became sub- 

 merged, while at the same time the island tilted so that the platform 

 sank to a sUghtly greater depth on the southeast than along the north 

 shore. Then reefs grew around the island, and on the north shore, 

 where the platform was shallow, the fringing reefs largely fused with the 

 off-shore barrier reef; but on the south shore, where the platform was 

 submerged more deeply, the fringing reef in most places did not fuse 

 with the barrier reef, but the lagoon remained intact between them. 

 Later a submiergence of about 190 feet occurred and the sea cliff ed the 

 shores, and finally the sea-level sank about 10 feet and then the modern 

 fringing reefs began to grow outward from the shores over the seaward 

 slopes of the island. 



Captain Frank A. Potts, of Trinity Hall, Cambridge University, 

 England, made a special study of the growth-rate of invertebrates other 

 than corals and finds, according to his report, that Lepas and Teredo 

 in Pago Pago Harbor, at 26° to 30° C. grow much more rapidly than 

 they do in temperate regions, but in other forms, such as hydroids 



