Daly — Pleistocene Glaoiation and Coral Beef Problem. 299 



time for renewed emphasis on the vital connection between 

 Pleistocene glaoiation and the conditions of Pleistocene inter- 

 tropical seas. 



Effect of Pleistocene glaoiation on intertropical sea-level. — 

 It is becoming increasingly apparent that the southern hemi- 

 sphere was affected by Quaternary glaciation contempoi-ane- 

 ously with the northern hemisphere.* At the time of their 

 maximum extension the ice-sheets, which Irave since melted 

 away, covered a total area which may be estimated as from five 

 to eight millions of square miles. One will not go far astray 

 in assuming six millions of square miles as the area thus degla- 

 ciated on the earth as a whole. This is one twenty-fourth of 

 the present area of the ocean. 



The average thickness of the ice cannot yet be determined, 

 but its order of magnitude can be stated. The depth and sur- 

 face gradient of the Labrador ice-cap were sufficient to drive 

 that sheet over the mountains of New England and jSTew York 

 state, and to give the effluent ice-streams crossing the Torngat 

 mountains of northeastern Labrador thicknesses of more than 

 half a mile.f The writer has found the maximum thickness of 

 the Cordilleran ice-cap of North America at the 49tli parallel of 

 latitude — near the southern limit of the cap — to be over 6,000 

 feet, and the average thickness about 2,500 feet. The ice-cap 

 of northwestern Europe varied between 1,500 feet in thickness 

 at the Harz mountains to "perhaps between 6,000 and ^,000 

 feet " in thickness over Scandinavia, and " the sheet may have 

 been as much as 4,000 or 5,000 feet thick in the northern part 

 of Britain.":}: Penek has estimated that the average thickness 

 of the Pleistocene ice was well over 1,000 metei's.§ 



The removal of enough water to form these great sheets of 

 ice would tend- to lower sea-level all around the globe by the 

 amounts here approximately stated : 



Estimated average thickness of ice 



(in feet) 3,000 3,600 4,000 5,000 



Corresponding decrease of ocean'' s 



depth {in ieet) 125 150 167 208 



Woodward, Hergesell, and others have shown that a second 

 cause for a negative movement of sea-level in the equatorial 

 zone is to be found in the sravitative power of the ice. Using 

 Woodward's formulas, it may be calculated that, if the ice had 

 an area of 6,000,000 square miles and an average thickness 



* Cf . H. Hess, Die Gletscher, Braunschweig, 1904, p. 396. 



f See Bulletin, Museum of Comparative Zoology at Harvard College, vol. 

 sxxviii, p. 351, 1903. 



I Sir Archibald Geikie, Textbook of Geology, vol. ii, p. 1305, 1903. 



g A. Penck, Separatabdruck, Jahrbuch Geog. Gesell. zu Miinchen, vii, 

 p. 30, 1883. 



