GLACIAL-CONTROL THEORY OF CORAL REEFS. 233 



shoals seem to be of volcanic origin. Rising from a sea bottom, 

 3,000 m. to 7,000 m. deep, each volcano is very high in absolute 

 measure and is also of notable area. The local extravasation of so 

 much lava may well entail local, moderate sinking of the earth's crust. 

 It is, indeed, possible that such sinking is very often caused directly 

 by volcanic action on a large scale. ^* More certain is the fact that, 

 after a great volcanic cone is formed, its upper and central part tends 

 to subside. Probably owing to the slow compacting of its deeper 

 tuffs and vesicular flows, as these are gradually heated and so softened 

 by the magma of the central vent, the mass slowly settles. The loss 

 of connate water in the deeper ash-beds, both because of the heating 

 and because of mere dead-weight, is another cause for settling in a local 

 area, below the highest part of the volcano. Actual subsidence of 

 this kind is exemplified in the "volcanic sinks" located at many 

 central vents of the first rank.^^ 



Possibly, therefore, some of the drowned valleys and other physio- 

 graphic features showing submergence of volcanic islands are to be 

 explained by local sinking to the extent of a few meters or a few scores 

 of meters. Clearly such subsidences would have very different 

 geological dates, according to the respective times of preliminary 

 volcanism. Erosion valleys so drowned in pre-Glacial periods would 

 be filled, below sea-level, with sediment, which would be cleaned out 

 again by the Pleistocene waves. Conceivably, some broad bays in 

 the existing central islands may have thus originated. 



However, Gerland's version of the subsidence theory does not 

 account for the essential contemporaneity of the present atoll, barrier, 

 and fringing reefs. That they have been developed nearly or quite 

 in the same interval of time is shown by their sectional areas and their 

 volumes, as measured, in each case, above the break of slope at the 

 platform on which the crowning reef stands. If the simultaneous 

 submergence of coral islands in general were really due to crustal 

 subsidence, the Darwin-Dana postulate seems to represent the only 

 possibility. Their view is the one now to be briefly examined. 



Uniformity of the Assumed Subsidence. As just hinted, the surface 

 outcrops and volumes of the greater barrier and atoll reefs, measurefl 

 from the levels of the lagoon floors, are respectively nearly equivalent 

 in the Pacific and Indian oceans and in the neighboring seas. If these 

 reefs were formed by subsidence, the earth's crust must have sunk at 



64 R. A. Daly, Igneous Rocks and Their Origin, New York, p. 185 (1914). 



65 R. A. Daly, ibid., p. 150. 



