610 
Geology. — “The Coral reef problem and TIsostasy”. By Prof. Dr. 
G. A. F. MoOLENGRAAFF. 
Translated from the Dutch, somewhat revised and augmented. 
(Communicated in the meeting of June 24, 1916). 
The question of the origin of coral islands (barrier reefs and atolls) 
has of late been brought to the foreground again by the recent 
publications of Darry *) and Davis *). 
It is generally known that according to Darwin’s theory a consi- 
derable subsidence of the floor of the ocean over extensive areas 
is one of the necessary conditions for the formation of barrier reefs 
and atolls. While Darwin’s theory has maintained itself splendidly, 
at any rate in its main principle, against the numerous and manifold 
objections raised against it more especially after the Challenger 
expedition, still the great crustal movements which it requires always 
remained its weakest point which did not meet with general approval. 
Darwin *) speaks expressly of real subsidence of the floor of the 
ocean itself, from which the coral formations, barrier reefs and 
atolls, rise. This real subsidence is very considerable; Dara *), 
in connection with Darwin’s theory of the coral islands, assumed 
that since the tertiary period in the Pacific alone a region extend- 
ing over 15 to 30 million square kilometres must have sunk 1000 
to 1600 metres. 
Depressions on such a scale of a considerable portion of the earth’s 
crust, although not impossible, are not very likely °). They have 
1) R. A. Dany. Pleistocene glaciation and the coral reef problem. Amer. Journ. 
of Science 4, XXX, p. 297, 1910, and The glacial-control theory of coral reets. 
Proc. of the Amer. Acad. of Arts and Sciences 51, p. 157, 1915. 
2) W. M, Davis. A SHALER memorial study of coral reefs. Amer. Journ. of 
Science 4, XL p. 223, 1915. 
W. H. Davis. The origin of corai reefs. Proc. Nat. Acad. of Sciences. I. p 146, 1915. 
— Extinguished and resurgent cora! reefs. Proc. Nat. Acad. of Sciences. IL. p. 
466, 1916. 
— The origin of certain Fiji atolls. Proc. Nat. Acad. of Sciences. IL. p 471, 1916. 
— Problems associated with the study of coral reefs. The Scientific Monthly. 
IL. p. 813, 1916. 
3) Cr. DARWIN. On the structure and distribution of coral reefs. Chapters V and VL. 
4) J. D. Dana. Manual of Geology. Fourth, last edition 1896, p. 350. 
5) It should be remarked that for the greater, central part of the Pacific all 
that is known until now about the relief of the bottom, tends to prove that this 
downward movement, if it has taken place at all, could not well liave been 
compensated by a more or less equivalent upheaval of regions of comparable 
extent, since the atolls and islands encircled by barrier reefs in this ocean appear 
to stand on somewhat elevated strips of the bottom of the ocean, which are 
surrounded by still deeper basins. Compare: Max Grou. Tiefenkarten der Oceane. 
