476 ROLLIN D. SALISBURY 



ing land, during the time necessary for furnishing the salt, at the 

 present rate. If some of the salt has been derived from the lithosphere 

 beneath the sea, the figures should be correspondingly reduced, 

 though it is probable that the land has furnished much more salt than 

 the sea bottom has. The reduction on this account is, in some meas- 

 ure, or perhaps altogether, offset by the allowance which should be 

 made in the opposite direction for the salt which has been deposited 

 among the sedimentary rocks of the earth. 



Amount of average rock decomposed. — From the amount of salt in 

 the sea, and from the amount of calcium carbonate which the sea is 

 estimated to have had, calculations may be made as to the amount 

 of average rock which must have been destroyed to produce them. 



i. The average composition of accessible non-sedimentary rocks 

 has been determined, probably with a fair degree of accuracy. 1 

 Knowing the percentage of sodium in this average rock, the volume 

 of rock which must have been decomposed in order to furnish the 

 sodium necessary to make the salt of the sea may be calculated. 

 The average rock contains about 2.53 per cent, of sodium, and of 

 rock containing this amount of sodium nearly 55,000,000 cubic miles 

 would need to be decomposed, to yield enough sodium to form the 

 salt now in the sea. The salt of the sea, therefore, seems to imply 

 the decomposition of some such quantity of average rock. Since 

 only about 23,500,000 cubic miles of rock now remain above sea- 

 level, 2 and since much of this is sedimentary rock derived from the 

 original rock, and since much of the non-sedimentary rock is not 

 decayed, and still holds its sodium, it would appear that the larger 

 part of the 55,000,000 cubic miles of rock necessary to yield the 

 requisite amount of sodium must have been removed from the land 

 to the sea. 



2. A similar line of inquiry may be based on the amount of calcium 

 carbonate which the sea has had in solution. The average rock 

 contains about 4.90 per cent, of CaO. 3 On p. 471 the figures 420, 

 738, and 850 feet were deduced as perhaps representing, as well as 

 they are now known, the limiting average thicknesses of limestone 

 material for the earth. The largest of these figures is about twice 



1 F. W. Clark, Bulletins 1 68 and 228, U. S. Geological Survey. 

 ^Murray, op. cit., Vol. IV, p. 40. 3 Clark, op. tit. 



