611 
remained a moot point and the papers published by Darry and Davis 
mentioned above deal mainly with this point, Davis’ conclusion being 
that subsidence on a considerable scale, as postulated by the theory 
of Darwin-Dana, is indispensable, while Dany on the contrary gives 
and upholds an explanation of the origin of coral islands in which 
subsidence of the floor of the ocean is „ot put forward as a necessary 
condition. 
Davis, in his paper, has worked out a point of view to which 
Dana had first drawn attention. Dana’), independently, derived a 
strong argument in favour of Darwin’s theory and one which Darwin 
himself had not yet used, from the contours and the relief of the 
islands which are surrounded by barrier reefs, and especially from 
(lie submerged or “embayed” valleys occurring on those islands. 
Davis tested and verified the value of this argument by investi- 
gations in a large number of coral islands in the Pacific and proved 
that where barrier reefs occur, the fringed coasts are indented almost 
without exception and possess “embayed valleys”. 
Finally Davis by clear reasoning’) arrives at the result that only 
the subsidence-theory of Darwin and Dana leads to a satisfactory 
solution of the problem of the origin of barrier reefs and atolls. 
Dary maintains, more emphatically than Prnck’) had done 
before, that in the pleistocene period the storing of large quantities 
of water in cireumpolar icecaps must have lowered the sea-level 
in aequatorial regions as much as 50 to 60 metres, which caused 
a corresponding equal lowering of the plane of abrasion and of 
the final base-level in those regions, whereas after the close of the 
glacial period a rise of the sea-level must have occurred of about 
the same amount caused by the melting of those icecaps. During 
this last rising of the sea-level or apparent subsidence of the land 
according to Dary and on account of it, the formation of the barrier 
Veröff. des Inst. für Meereskunde. NF. A. 2. 1912. A non-compensated subsidence 
of such a considerable part of the Pacific since the tertiary period would have 
caused an apparent general rise of all continents — calculated from their present 
size — of about 120 metres. One might expect to find at least some indication 
of such an upheaval, but this is not the case. 
1!) J. D. DANA, in United States WiLKEs’ Explor. Expedition, Geology p. 131, 1849, 
and W M. Davis. Dana’s confirmation of Darwin’s theory of coral reefs. Amer, 
Journ. of Science 4. XXXV p. 183, 1913. 
2) Very interesting in this respect is Davis’ preliminary paper, entitled: The 
Home Study of Coral Reefs. Bull. Amer. Geogr. Soc. Vol. XLVI. p. 561, 1914, | 
5) A. PencK, Morphologie der Erdoberfläche III, p. 660, 1894. 
Prenck thinks it possible that during and on account of the pleistocene glaciation 
the sea level has been lowered as much as 150) metres. 
39* 
