THE MINERAL MATTER OF THE SEA 477 



the smallest. For convenience of calculation, and in view of the some- 

 what uncertain nature of the data on which they are based, no further 

 error of great magnitude will be involved if the smallest of these 

 figures be regarded as half of the largest, that is, 425. 



If 425 and 850 feet be assumed to represent the maximum and 

 minimum average thickness of limestone material, as nearly as it is 

 now known, and if four-fifths of this be assumed to be calcium car- 

 bonate, 1 the amounts of calcium carbonate in the earth's crust would 

 be equivalent to nearly 12,700,000 cubic miles in the one case, and to 

 25,400,000 cubic miles in the other. These figures are much smaller 

 than that given on p. 475, but that includes the calcium carbonate 

 re-dissolved, and carried anew to the sea, after being once deposited. 

 These do not. 



The amounts of average rock the decomposition of which would 

 be needed to yield the amount of calcium necessary for these amounts 

 of calcium carbonate, supposing all the calcium to be separated so as to 

 be available for union with carbonic acid gas, would be 145,000,000 

 cubic miles and 290,000,000 cubic miles, respectively. These results, 

 it will be seen, are about 2 . 6 and 5 . 2 times as large, respectively, 

 as those derived from the calculation based on salt of the sea. Even 

 after allowance is made for the salt which has been deposited, the 

 figures are far apart. This means that some of the sodium does not 

 unite with chlorine to produce salt, or that more of the sodium chloride 

 has been deposited than is known, or that there is some other dis- 

 crepancy in the data. In spite of the discrepancy, however, it will 

 be noted that the figures belong to the same order of magnitude. 



The same problem may be approached in another way. We have 

 seen that the amount of calcium carbonate which should have been 

 carried to the sea in the time necessary for the accumulation of the salt, 

 assuming all of the latter to have come from the land, and at the 

 present rate, is more than 68,000,000 cubic miles, and, if the calcium 

 sulphate be added, about 76,000,000 cubic miles. To yield the calcium 

 called for by these volumes, the decomposition of about 777,000,000 

 cubic miles of average rock would be necessary, if the calcium car- 

 bonate only is taken into consideration, and more than 60,000,000 



1 The rest being magnesium carbonate. Clark, Bulletin 228, U. S. Geological 

 Survey, pp. 20, 21. 



