194 CARNEGIE INSTITUTION OF WASHINGTON. 



tion of the boriDg the coral had undergone the changes already observed in 

 material from the previous borings, which had led to a decided change in the 

 color and, to some extent, in the chemical constitution, the original white 

 coral having been replaced to a great extent by extraneous material. Whether 

 or not this change has taken place after a partial solution of the original skele- 

 tal mass is not apparent from a superficial examination of the core and can be 

 determined only after an examination of sections, and possibly after chemical 

 analysis. 



The Geological Interpretation of the Coral Reefs of Tutuila, Samoa, 

 by Rollin T. Chamherlin. 



The origin of coral reefs having long been a topic of much debate, one of the 

 objects of this expedition to Samoa was an intensive study of the reefs of the 

 island of Tutuila, for the fight which they might throw upon this controverted 

 question. For the correct interpretation of these reefs a knowledge of the 

 physical history of the island was deemed essential. In an endeavor to un- 

 ravel the sequence of events and the stages in the growth and development of 

 the island, three weeks were spent on Tutuila, in July 1920, investigating the 

 shore-lines, the erosion profiles, and other physiographic and geologic features 

 which are the present keys to the geologic past. 



The island of Tutuila is a volcanic pile whose slopes have been attacked by 

 the sea until a broad wave-cut platform, 2 miles in width, has come to surround 

 the island. This broad shelf of planation, originally cut in the volcanic rocks 

 not far below sea-level, now lies at least (though probably not much more than) 

 400 feet below sea-level. Streams on land cut deep valleys adjusted to the 

 shores about 400 feet below the present strand-line. On the outer margin of 

 the wave-cut platform, corals commenced to build a barrier reef, while a fring- 

 ing reef grew outward along the shore. The island seems to have sunk some- 

 what relative to the sea, accompanied by tilting which submerged the south 

 side more deeply than the north side. In the shallower water on the north 

 side of the island the fringing reef, owing to the rapid growth of coral there, 

 has in places become merged with the barrier reef. On the south side, where 

 the water was deeper, the reefs are narrower. Subsequently the island became 

 progressively submerged, though there may have been several halts and oscil- 

 lations in this general movement. The last important strand-line before the 

 present level was attained stood somewhat more than 10 feet above the present 

 high tide, as evidenced by the conspicuous wave-cut benches seen on nearly 

 all the headlands of the island. 



Tutuila, therefore, is consistent with the Darwin-Dana coral-reef hypothesis 

 to the extent that a submergence of 400 feet has occurred since the corals 

 began to form the old barrier reef; but in other respects it does not fit the re- 

 quirements of that hypothesis, inasmuch as the barrier reef, instead of being 

 built up several thousand feet from the slopes of a sinking island, is found to 

 be rooted on a broad, wave-cut platform which, shghtly submerged, afforded 

 favorable conditions for coral growth. 



With respect to the glacial-control hypothesis, the shelf 400 feet below pres- 

 ent sea-level is more deeply submerged than that hypothesis would require. 

 If the broad platform were in reality cut during the comparatively brief stages 

 of maximum glaciation and consequent low level of the oceans, according to 

 the glacial-control hypothesis, a subsequent additional relative rise of the sea 

 of some 200 feet would still be necessary. This is possible, but the assump- 

 tion must necessarily take away much of the force of the glacial argument as 

 applied to Tutuila. The lowered level of the oceans during the glacial epochs 

 of the Pleistocene must inevitably have had its effects on the shores, but the 



