GLACIAL-CONTROL THEORY OF CORAL REEFS. 171 



Atlantic and the character of the West Indian coral fauna is not 

 solved, but it should be attacked, not only for its zoological significance 

 but also as a future, more complete test of the Glacial-control theory 

 of atolls and barrier reefs in general. 



Lowering of Sea-level by Pleistocene Glaciation. 



Diminished Volume of Ocean Water. The second main postulate 

 of the Glacial-control theory is likewise difficult to state quantitatively 

 other than in terms of an order of magnitude. The important Pleisto- 

 cene ice-caps were five in number. Each of them meant a diminu- 

 tion of the volume of the ocean water and also a gravitative lowering 

 of sea-level in the tropical seas. If all the ice-caps reached maximum 

 size simultaneously, and if the area, location, and average thickness 

 of each were known, it would be a rather simple matter to compute 

 the position of Pleistocene sea-level in relation to present sea-level. 

 A similar degree of certainty as to relation of the former level to Plio- 

 cene sea-level would still not be attainable, since the qviestion as to the 

 size, and even the existence, of an Antarctic ice-cap during Pliocene 

 time has not yet been answered. 



The depth of water off the edge of the ^Antarctic ice-cap is so great 

 that one cannot assume a Pleistocene area for that sheet much larger 

 than its present area (about 13,000,000 square km. or 5,000,000 square 

 statute miles). ^° The probable maximum is about 16,000,000 square 

 km. The present average thickness of the ice is, of course, unknown; 

 the minimum possible estimate is doubtless 300 m. In a personal 

 letter. Professor T. W. E. David states that the Great Ice Barrier has 

 recently decreased much more than 600 feet (183 m.), probably 1,000 

 feet (305 m.), while the Beardmore glacier, a distributary of the ice- 

 cap, has clearly shrunk 2,000 feet (610 m.) vertically. The least 

 assignable average thickness for the whole ice-cap at its greatest 

 strength is probably 600 m., and it may have been three times as great. 



The present area of the Greenland ice-cap is about 1,900,000 square 

 km. It may not have been much bigger in the Glacial period; the 

 greatest area of its non-floating portion was probably less than 

 3,000,000 square km. The present average thickness and the average 

 thickness at maximum strength are alike unknown, but respectively 



20 Any submerged portion of the ice-cap is negligible in connection with the 

 main problem. 



