14 SMITHSONIAX MISCELLAXEOUS COLLECTIOXS VOL. 56 



In the ocean, as previoush' shown, there are l-il30 X 10^- metric tons 

 of sodium. This amount would be supplied by the complete decomposi- 

 tion of 48,225,000 cubic miles of an igneous rock having the composition 

 shown by Clarke's average, or 42,385,000 cubic miles on the basis of 

 Washington's figures. Some sodium, however, is retained by the sedimen- 

 tary rocks, for the reason that decomposition is rarely, if ever, perfect. 

 Fragments or particles of feldspar are especially common in the sedimen- 

 taries. On comparing the average composition of the latter with that of 

 the igneous rocks, assuming that ahimina has remained constant, the 

 following ratios appear : 



Clarke. Washington. 



Na20 retained, per cent 1.29 1.38 



NaaO lost, per cent 2.12 2.50 



3.41 3.88 



Hence, for the total volume of igneous rock needed to furnish all the 

 sodium of the ocean and of the sediments, assuming that the sodium lost 

 represents the marine portion, we have : 



Prom Clarke's average 77,570,000 cubic miles 



From Washington's average. 65,782,000 " " 



These figures correspond, respectively, to a shell of igneous rock, com- 

 pletely enveloping the globe, 2050 feet thick in one case, or 1740 in the 

 other. For present purposes the higher estimate will be taken, in order 

 that the deductions may be maxima. 



If, now, the sedimentary rocks were of the same volume as the igneous 

 rocks from which they were derived, and if the sandstones form 15 per 

 cent, of the 77,570,000 cubic miles, the bulk of the latter will be, very 

 roughly, 10,635,500 cubic miles. The assumptions thus made are of 

 course not exact, but they will suffice for the computations now in hand. 

 The errors are negligible, so far as present needs are concerned. Assume 

 further that the sandstones, the most porous of the rocks, contain an 

 average pore space of 20 per cent., or 2,127,100 cubic miles, and that all 

 of it was once filled by sea water, representing 99,861,000,000,000 metric 

 tons of sodium. If all of that sodium were now present in the sandstones, 

 and erosion began at the rate now assigned to the rivers, namely, 175,- 

 040,000 tons of sodium annually, the entire accumulation would be re- 

 moved in 570,000 j^ears. This, compared with the estimate already 

 reached for geological time, is an almost negligible quantity. Even if the 

 entire volume of the sedimentary rocks carried the same proportion of 

 sodium as is here assigned to the sandstones, it would be exhausted in 

 about 3,800,000 3^ears. The con-ection for disseminated salt is, therefore, in 

 all probability small, and not likely to exceed 1 per cent. 



