560 



SCIENCE 



[N. S. Vol. LIII. No. 1382 



is consistent Tvith the Darwin-Dana coral-reef hy- 

 pothesis 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 requirements of that hypothesis, inas- 

 much as the barrier reef, instead of being built 

 up several thousand feet from the slopes of a 

 sinking island, is foimd to be rooted on a broad, 

 ■wave-cut platform. 



Dr. Mayor commeiits on Chamberlin's state- 

 ment in part as follows : 



Professor R. T. Chamberlin, who made a spe- 

 cial study of the relation between the reefs and 

 the volcanic shores of the island, . . . finds that 

 the ancient barrier and fringing reefs which once 

 surrounded the island and are now drowned grew 

 upon a platform which had been cut by the sea 

 and afterwards submerged and not upon the un- 

 altered slopes of the island. Thus the Darwin- 

 Dana theory does not apply to Tutuila. 



Chamberlin's summary concerning the ori- 

 gin of the reefs is excellent as far as it goes, 

 and it is to be presumed that if he publishes a 

 fuller account of his results he will then sup- 

 plement the present brief statement with an 

 explanation of the conditions which deter- 

 mined that Tutuila should be for a time reef- 

 free and therefore exposed to abrasion before 

 it became reef -encircled, and with a descrip- 

 tion of the high cliffs that must have risen 

 at the back of the now submerged 2-mile plat- 

 form and of their relation to the recently cut 

 cliffs the base of which is close to actual 

 sea level. 



But excellent as the present summary is 

 with respect to the reefs of Tutuila, neither 

 the passage above quoted from it nor the 

 passage quoted from Mayor's comment upon 

 it does justice to Darwin's theory of coral 

 reefs; for in so far as the quoted passages im- 

 ply that the submerged barrier reef of Tutuila 

 does not exemplify the " Darwin-Dana " 

 theory, they hold good only for an imperfect, 

 indeed an incorrect conception of that theory. 

 As a matter of fact the Tutuila reefs, both 

 submerged and at present sea level, exemplify 

 certain special phases of Darwin's theory in 

 a remarkable manner, as the following cita- 



tions from his " Structure and Origin of Coral 

 Eeefs " (1842) will make clear. 



In the first place, Darwin nowhere asserted 

 that barrier reefs must be " built up several 

 thousand feet from the slopes of a sinking 

 island," or that they could not be built up 

 from a " broad, wave-cut platform," as Cham- 

 berlin implies, or that they must grow up from 

 " the unaltered slopes of an island," as Mayor 

 assumes. All that Darwin's theory of barrier 

 reefs and atolls demands is that a foundation 

 of any form shall subside slowly enough for 

 the reef to grow upward and maintain its 

 surface at sea level. The form of the foun- 

 dation is immaterial. It is true that the 

 typical island profile which Darwin drew in two 

 figures (pp. 98, 100), to represent a subsiding 

 foundation on which a fringing reef would 

 be transformed into a barrier reef and a bar- 

 rier reef into an atoll, showed an island of a 

 particular form, as graphic illustrations al- 

 ways must; but as this profile was modeled 

 upon that of the island of Bolabola, a deeply 

 denuded member of the Society group, it ef- 

 fectually disposes of Mayor's assumption that 

 Darwin thought reefs grew up from " the un- 

 altered slopes of an island." 



It is true that Darwin nowhere wrote any- 

 thing about the denudation of Bolabola, but 

 he was perfectly familiar with the fact that 

 the slopes of volcanic islands are altered by 

 erosion and abrasion. His geological philos- 

 ophy was somewhat primitive, for he thought 

 that many volcanic islands had been uplifted 

 after their conical form had been produced by 

 eruption, and that during the resulting emer- 

 gence the sea cut valleys in the island slopes; 

 it was, indeed, by this process that he accounted 

 for the repeated breaching of certain original 

 " basaltic rings," composed of outward dipping 

 lava beds, and their conversion into a circuit of 

 separated hills, such as characterize the islands 

 of " St. Jago " in the Cape Verde group, St. 

 Helena, and Mauritius. He also knew that 

 " deep arms of the sea . . . penetrate nearly 

 to the heart of some [reef] encircled islands," 

 Eaiatea in the Society group being mentioned 

 as one of them; and the depressions occu- 

 pied by such sea arms were surely understood 



