478 ROLLIN D. SALISBURY 



cubic miles more, if the calcium sulphate be included. Seven hundred 

 and seventy-seven million cubic miles is more than 33 times the 

 estimated cubic contents of the land, while 837,000,000 cubic miles 

 (777,000,000 + 60,000,000) is about 36 times the estimated cubic con- 

 tents of the land. These figures should be reduced to make allow- 

 ance for the calcium carbonate which has been redissolved after having 

 been precipitated. If half the calcium carbonate which the ocean 

 water has had has been re-dissolved after having been once precipi- 

 tated, the last figures should be divided by 2. Reduction should also 

 be made for the calcium carbonate which has been derived from the 

 rocks beneath the sea. 



Assuming that 2 is the proper divisor, the amount of average 

 igneous rock which must have been destroyed to produce the amount 

 of calcium carbonate carried to the sea, at the present rate, in 370,- 

 000,000 years, is about 388,500,000 cubic miles, or, if calcium sulphate 

 be included, 418,500,000 cubic miles. This amount of rock would 

 make a layer more than 2 miles thick over the entire surface of the 

 earth, and more than 6^ miles thick over the continents and conti- 

 nental shelves. This volume of mineral matter is about 18 times all 

 that is now above the surface of the sea. These calculations, even 

 though the figures involve a considerable error, indicate that an 

 enormous body of rock must have been decomposed. 



By both these methods of calculation, based on the calcium car- 

 bonate, it will be seen that the amount of average rock needed (pp. 

 476 and 477) to yield the estimated supply of calcium carbonate is 

 considerably more than that needed to yield the known amount of 

 salt. It is probable that salt furnishes the better basis for the esti- 

 mates, since the amount which has been formed is probably more 

 nearly known, most of it being presumed to still remain in the sea. 



Even the above figures do not represent the full measure of transfer 

 of the material from land to sea. In the decomposition which igne- 

 ous rock undergoes, before yielding up its calcium in soluble form, it 

 undergoes notable expansion. It follows that the preceding figures 

 great as they are, may not represent the actual amount of average 

 rock material destroyed, and removed from land to sea. 



Assigning the average igneous rock a mineral constitution con- 

 sistent with its chemical composition, its expansion on decomposition 



