668 THE JOURNAL OF GEOLOGY. 



Professor James D. Dana estimates that the rate of increase 

 of coral reef limestone formations, where all is most favorable, 

 does not exceed perhaps a sixteenth of an inch in a year, or five 

 feet in a thousand years. Of this he says, "And yet such lime- 

 stones probably form at a more rapid rate than those made of 

 shells." 1 



Messrs. Murray and Irvine, in their valuable paper on coral 

 reefs and other carbonate of lime formations in modern seas, cal- 

 culate the total amount of calcium in the whole ocean to be 

 628,340,000 million tons; also they estimate that 925,866,500 

 tons of calcium are carried into the ocean from all the rivers of 

 the globe annually. At this rate it would take 680,000 years for 

 the river drainage from the land to carry down an amount of 

 calcium equal to that at present existing in solution in the whole 

 ocean. They say further: "Again, taking the 'Challenger' 

 deposits as a guide, the amount of calcium in these deposits, if 

 they be 22 feet thick, is equal to the total amount of calcium in 

 solution in the whole ocean at the present time. It follows from 

 this that, if the salinity of the ocean has remained the same as at 

 present during the whole of this period, then it has taken 680,000 

 years for the deposits of the above thickness, or containing 

 calcium in amount equal to that at present in solution in the 

 ocean, to have accumulated on the floor of the ocean." 2 Accord- 

 ing to this calculation the mean rate of accumulation over 

 existing oceanic areas is 68 \\ . or .000032 feet per annum. 



Was the Deposition of Chemical Sediment More Rapid in Paleozoic 

 Time? — It has been claimed that the quantity of lime poured into 

 the ocean in earlier times was greater than during the later epochs 

 of geological history, — this arising from the more rapid disin- 

 tegration of the Archean, crystalline and volcanic rocks. It is 

 undoubtedly a fact that the ocean was stocked in Archean and 

 Algonkian time with matter in solution that produced salinity, 

 but we have no evidence from chemical precipitation that more 



1 Corals and Coral Islands, 3rd Ed., 1890, pp. 396-397. 

 2 Proc. Royal Soc, Edinburgh, Vol. 17, 1890, p. 101. 



