THE MINERAL MATTER OF THE SEA 475 



period of 84,000 years should be lengthened. Treating calcium car- 

 bonate and calcium sulphate as one, it would take the rivers about 

 740,000 years to contribute what the sea now contains. 



The reason for the great discrepancy between these figures and 

 those which represent the time necessary for the accumulation of the 

 salt of the sea, is doubtless found in the fact that the calcium com- 

 pounds are extracted by the organisms of the sea, to make shells, 

 tests, etc., about as fast as they are brought in, while the salt remains 

 in solution. 



The amount 0} calcium carbonate which may have been in solution 

 in the sea.— The amount of calcium carbonate which would have been 

 carried to the sea by rivers in 370,000,000 years, at the rate at which 

 rivers are now contributing it, is about 68,600,000 cubic miles. In 

 the same time the rivers should, at their present rate, have contributed 

 about 7,200,000 cubic miles of calcium sulphate, or nearly 76,000,000 

 cubic miles of calcium carbonate and calcium sulphate. Since not 

 more than about 179,000 cubic miles of calcium compounds (car- 

 bonates and sulphates) remain in solution in the sea at the present 

 time, it will be seen that an enormous amount must have been 

 extracted. 



Most of the calcium deposited in the sea has been deposited in 

 the form of calcium carbonate. If an amount of calcium carbonate 

 and calcium sulphate corresponding to the difference between 76,000,- 

 000 and 179,000 cubic miles has been deposited, it would make a 

 layer about 1,920 feet thick over the entire earth. Since, however, 

 some of the calcium carbonate taken in solution to the sea has been 

 redissolved, some of it repeatedly, after precipitation, the average 

 thickness of that which has been deposited must be much less than 

 1,920 feet. Perhaps this figure should be reduced by one-half on 

 this account. 



Sixty-eight million six hundred thousand cubic miles, it may be 

 noted, is about three times the cubic contents of all lands. Assuming 

 that rivers have been supplying calcium carbonate and sodium 

 chloride to the sea at the present rate, and assuming that the land 

 has been the only source of these materials in the sea, it follows that 

 the rivers should have carried to the sea an amount of calcium car- 

 bonate equivalent to about three times the cubic contents of all exist- 



