14 PAPERS FROM THE DEPARTMENT OF MARINE BIOLOGY. 



Of the explanations so far proposed for living coral reefs, it seems that 

 those supported by Daly, Andrews, and Vaughan' furnish the best working 

 hypothesis. That there was withdrawal of water from the ocean to form 

 the Pleistocene ice-caps, and that the water so withdrawn was returned to 

 the sea with the melting of the glaciers seems undeniable. The amount of the 

 lowering and subsequent restoration of sea-level can be only approximated, 

 as the factors to be considered in making the estimates are not precisely 

 known. However, it is interesting to note that recent computations are 

 of the same order of magnitude. Thus W. J. Humphreys, at the request 

 of Vaughan, made an estimate showing that the amount of lowering of sea- 

 level was probably 67 meters,' while Daly in his recent paper {op. sup. cit.) 

 estimates it as between 50 and 60 meters. The two estimates accord very 

 closely. As the Glacial period is variously estimated as having been between 

 250,000 and 1,000,000 years in duration, there was time for considerable 

 marine planation. The studies of Vaughan^ on the physiography of the 

 sea-bottom in the West Indies and Central America, and Daly's and 

 Vaughan's profiles across the Australian continental platform and the Great 

 Barrier Reef accord with the demands of Daly's Glacial Control theory, as 

 evidence is adduced in favor of a Recent rise of sea-level by an amount 

 approximating 25 fathoms. All Pleistocene and Recent changes in sea-level 

 are surely not due solely to glaciation and subsequent deglaciation, but that 

 the more important ones were in part or largely caused by such phenomena 

 seems beyond doubt. 



Daly and Vaughan agree as to the superposition of the living reefs on 

 antecedent platforms, and both now agree that some platforms clearly ante- 

 date the Pleistocene, and thus their formation can not be referred to proc- 

 esses operative during that time; also both Daly and Vaughan agree that 

 there is strong evidence in favor of the margins of such old platforms having 

 been remodeled during Pleistocene time, while the sea stood at various posi- 

 tions lower than at present. The evident geologic antiquity of some plat- 

 forms does not signify that others are equally old, and it is highly probable 

 that many platforms were formed during the Pleistocene period. 



As Vaughan, Daly, Davis, and others have shown shore-line history needs 

 to be studied not only within coral-reef areas, but over other parts of the earth 

 where sea-level intersects the land; the depth to which submarine planation is 

 effective and its limiting conditions have not been adequately investigated; 

 and but little trustworthy information is available on the rates at which the 

 sea will cut away the land, and on the factors which determine the rates. 

 The complex of causes which may result in shift in the position of strand- 

 line, and the evaluation of the effects of each are among the problems before 

 us; and the rates at which the sea-bottom may be aggraded is unknown. 



'See especially his recent paper in Wash. Acad. Sci. Jour., 6, pp. 53-66, Feb. 4, 1916. 

 'Wash. Acad. Sci. Jour., vol. 5, pp. 445-446, June 19, 1915. 



