GEOLOGY AND PALEONTOLOGY OF THE CASUAL ZONE. 253 



it stood previous to the formation of the continental glaciers, unless 

 crustal changes in the earth counterbalanced the effects of such with- 

 drawal and return of oceanic water. Reference here will be made 

 to only the two latest computations. 



W. J. Humphreys, as part of a symposium before the Geological 

 Society of Washington, on March 24, 1915, said: 1 



The fact that the average thickness of the ice cap during the last glaciation can be 

 only roughly estimated renders any calculation of its effect on ocean level corre- 

 spondingly doubtful. It does not seem probable, however, that they should have 

 averaged much if any thicker than the present caps of Greenland and of Antarctica, 

 which a number of good observers have estimated to be about 1,000 meters. Taking 

 this value and assuming the deglaciated area to be equal to one-fifteenth the area of 

 the ocean, or, roughly, twice the glaciated area of North America, we estimate the 

 change in sea level to have been about 67 meters. As already stated, this is only 

 an estimated change, but perhaps it is a conservative estimate. 



Daly in his paper on the Glacial-control theory of coral reefs sum- 

 marizes his discussion in the following words: 2 



Combining results, it is seen that, at the time of maximum glaciation, the tropical 

 seas probably had an average level which was 60 to 70 meters (33 to 38 fathoms) lower 

 than at the present time. 



The estimates of Humphreys and Daly are essentially the same. 



As maximum glaciation was probably not of long duration the 

 greatest effect of submarine terracing would be expected in some- 

 what shallower depths, probably between 20 and 30 fathoms. 



EATE OF GROWTH OF CORALS AND LENGTH OF POST-GLACIAL TIME. 



Recently I have published two summaries of the results of my 

 experiments and observations on the growth rate of Floridian and 

 Bahamian corals, and compared my results with those obtained by 

 investigators in the Pacific. 3 The following statements are taken 

 from the second of the papers referred to in the footnote: 



As has been stated, the primary object of this investigation was to get an approxi- 

 mate measure of the rate at which corals might build reefs. In order to make this 

 estimate the true reef corals must be considered separately from those which live in 

 other habitats. The reef species par excellence in the Recent and Pleistocene reefs of 

 Florida and the West Indies is Orbicella annularis; after it in importance are Maean- 

 dra strigosa, M. labyrintkiformis, and Siderastrea siderea. Other corals, the most impor- 

 tant of which is Pontes astreoides, with Agaricia and Favia fragum of secondary 

 importance, occur in the areas intermediate between the prominent heads. In some 

 areas Acropora palmata is the dominant species. The massive heads form the strong 

 framework of the reef, with infilling by other corals and other organisms. Therefore 

 the upward growth rate of Orbicella annularis on the reef is critical. * * * 



1 Humphreys, W. J., Changes of sea level due to changes of ocean volume, Washington Acad. Sci. 

 Journ., vol. 5, pp. 445-446, June 19, 1915. 



2 Amer. Acad. Arts and Sci. Proc, vol. 51, p. 174. 



3 Vaughan, T. W., Geologic significance of the growth-rate of the Floridian and Bahaman shoal-water 

 corals, Washington Acad. Sci. Journ., vol. 5, pp. 591-600, 1915; Growth rate of the Floridian and Bahaman 

 shoal-water corals, in On Recent Madreporaria of Florida, the Bahamas, and the West Indies, etc., 

 Carnegie Inst. Washington Yearbook Mo. 14, pp. 221-231, 1916. 



