254 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



Using these figures [in the paper referred to] as the basis of a further computation, 

 a reef by the continuous upward growth of corals [Orbicella annularis] might attain 

 at a rate of 6 mm. a year a thickness of 25 fathoms=150 feet in 7,620 years; and at a 

 rate of 7 mm. a year it might attain the same thickness in 6,531 years. 



Should the growth rate of Aa-opora palmata be taken as a measure, the time to 

 accumulate such a thickness would be considerably less. This species forms spread- 

 ing, palmate fronds, rising from stout bases. As age advances the fronds thicken and 

 can withstand the pounding of surf and breakers. The average upward growth is 

 between 25 and 40 mm. per year, but as the interspaces between the fronds are con- 

 siderable in volume, comparisons with Orbicella annularis must be based upon relative 

 increases in weight for a known period . * * * 



These two estimates [as shown in the paper cited] give a measure of the limits of 



reef formation under continuously favorable conditions for upward growth. Such 



corals as Orbicella annularis might form a reef 150 feet thick in between 6,500 years 



and 7,600 years; while such corals as Acropora palmata might form a similar thickness 



in 1,800 years. 



******* 



The data available for the Pacific corals are not so abundant as those for the Atlantic, 

 nor have the records, with few exceptions, the same degree of precision. However, 

 they are sufficient for some general comparisons. The general growth rate of branching 

 corals is nearly the same for both regions; but the growth of the massive forms in the 

 Pacific appears to be appreciably more rapid than that of similar forms in the Atlan- 

 tic. Therefore it seems probable that in the coral reef regions of the Pacific and Indian 

 oceans a reef 150 feet thick may form under favorable conditions in less than 6,000 

 years. According to Gardiner such a reef might form in 1,000 years. 



As the disappearance of the last continental ice sheets is estimated to have been 

 between 10,000 years ago in Scandinavia and Alaska and 40,000 years ago at Niagara, 

 the data presented show that there has been ample time for the development of any 

 known living reef since deglaciation. 



EFFECT OF LOWERING OF MARINE TEMPERATURE ON REEF CORALS DURING GLACIATION. . 



Daly in his paper on the Glacial- control theory devotes much atten- 

 tion to the probable extinction of reef corals over large areas and 

 their restriction to only the hotter parts of the ocean during glacia- 

 tion. 1 Daly's discussion of this subject is interesting and suggestive, 

 but not really convincing. It is one on which far more research is 

 needed. I rather hope that the data I have recently presented in 

 my paper on the temperature of the Florida coral-reef tract 2 will aid 

 in furnishing a basis for such a computation. That there was a 

 lowering of the vitality of corals over large areas marginal to tropics 

 can scarcely be doubted, but that reef corals thrived throughout 

 Pleistocene time appears more than merely probable. 



In this connection this following list of corals from the elevated reefs 

 of Barbados is pertinent. Professor Jukes-Browne sent the collection 

 to me after Prof. J.W. Gregory had published his paper on the Bar- 

 badian elevated-reef corals, 3 making the statement that great care 

 had been taken in determining the height above sea level at which 



1 Amer. Acad. Arts and Sci. Proc, vol. 51, pp. 166-171. 

 1 Carnegie Inst. Washington Pub. 213, pp. 319-339, 1918. 



8 Gregory, J. W., Contributions to the paleontology and physical geography of the West Indies, Geol. 

 Soc. London Journ., vol. 51, pp. 255-310, pi. 11, 1895. 



