TIO GiwEle SHIOVINEL, Mk. 
Were the various constituents of rocks equally susceptible to 
the action of drainage waters,they would be carried to the sea in 
the proportions in which they occur in the crust. That this is far 
from being the case is a most familiar fact, and it is evident that 
they go into solution at very different rates. These rates are 
expressed by the ratios between the amounts of the various con- 
stituents in the rocks and in the drainage waters, respectively. 
That these relative rates must vary widely in different cases there 
can, of course, be no doubt, but the average rates are simply 
determined by the comparison of the compositions of the surface 
rocks and of the drainage waters. For the latter, Clarke’s “‘ General 
Mean’ is taken, column I, using, as in all other cases, only those 
constituents that are abundant in rocks. 
For the rocks, the simplest method is to take Clarke’s average 
composition of the lithosphere,* but this introduces an obvious 
source of error. This average represents a shell ten miles thick, 
of which probably 95 per cent consists of igneous rock. Drainage 
waters, being relatively superficial, exert their influence on rocks 
that are probably 75 per cent sedimentary and have lost certain 
constituents, particularly sodium, now held in solution in the sea. 
For this reason, it is necessary to use, as the basis of calculation, 
figures that allow for this permanently dissolved material. More- 
over, in so far as the elements in sediments are combined differently 
from what they were in the crystalline rocks, their relative solubili- 
ties are doubtless affected. Correct results are to be obtained only 
by comparing the composition of river waters with that of the 
actual materials of their basins, and in the absence of precise data, 
the closest approximation possible must be sought. 
This has been done by taking Clarke’s “weighted mean”’ 
of the composition of sedimentary rocks and his estimate of the 
composition of igneous rocks* and combining them in the ratios 
of three to one, in accord with Von Tillo’s estimate of the relative 
areas of sedimentary and igneous rocks, the results appearing in 
LO PCs specs : 
2“The Data of Geochemistry,” U.S. Geol. Surv. Bull., No. 330, p. 31. 
3F. W. Clarke, A Preliminary Study of Chemical Denudation, Smithsonian Misc. 
Coll., Vol. LVI, No. 5, p. 13, 1920. 
4 Loc. cit. 
