300 Daly — Pleistocene Glaciation and Coral Meef Problem. 



vuryine: from 3,000 to 5,000 feet, tlio attraction of the ice would 

 lower the level of the equatorial sea by amounts rangin<:; from 

 five to eight fathoms.* 



Taking the two effects together, the formation of the ice- 

 siieets (w-hich have since disappeared) would produce a nega- 

 tive movement of sea-level in low latitudes to an amount rang- 

 ing between twenty-five and forty-five fathoms. Assuming 

 3,(i00 feet as the average tiiickness of the ice, the shift of level 

 in the equatorial sea would be about thirty fathoms. f Con- 

 versely, the deglaciation of the full 6,000,000 square miles 

 would raise the level in the equatorial zone by about thirty 

 fathoms. 



The Plateaus. — Can we connect the later, positive, movement 

 of level with the forms of existing annular and barrier reefs ? 

 That qiiestion suggests another, as to the origin of the bro'ad 

 plateaus on which the reefs have been built. 



A principal datum for the discussion of this subject must be 

 the range of depths of the water above the plateaus. An 

 approximation to those depths is given in the depths of the 

 atoll lagoons and of the barrier channels. It is clear, however, 

 in spite of Murray's hypothesis of solution as explanatory of 

 lagoons, that the vast majority of lagoons and barrier channels 

 are slowly filling ujj ; so that the average depth of the plateau 

 is somewhat greater than the average depth of the deeper part 

 of lagoon or channel.:}; In the case of many a small atoll, 

 which, on account of limited size, has been nearly filled with 

 calcareous deposits, the plateau is many fathoms below the 

 deepest hole in the lagoon. 



The following table (col. 1) gives the maximum depths of 

 water charted in representative atoll lagoons and barrier chan- 

 nels of the Pacific and Indian oceans. § In some cases the 

 depths are given for great plateaus bearing relatively few reefs. 

 These depths are usually greater than the maximum depths in 

 atoll lagoons and barrier channels, but are of the same order 

 of magnitude. Column 2 shows the estimated average depths 

 of the deeper and generally considerable parts of lagoons or 

 channels. 



* R. S. Woodward, Bull. U. S. Geol. Survey, No. 48, 1888, pp. 41, 70, etc. 



•|- Penck's estimate of 150 meters for this change of level seems too high, 

 even if the volumes of the present Antarctic and Greenland ice-ca23S be 

 included in the computation. 



XCi. W. J. Sollas, in " The Atoll of Funafuti," 1904, pp. 6 and 27 ; and 

 E. Langenbeck, Die Theorieen Tiber die Entstehung der Koralleninseln, 

 Leipzig, 1890, p. 50 S. 



§ Nearly all figures in this table have been derived from Agassiz's very 

 convenient reproductions of Admiralty and other charts, to be found in Bul- 

 letins Xos. 28 and 33, and Memoirs Nos. 28 and 29, of the Museum of Com- 

 parative Zoology at Harvard College (1898-1903). 



