108 Cat SVR aie 
the solid is treated with a definite amount of the solvent, equilib- 
rium is established, and there is no further change of composition 
of either solid or solution. In nature, on the other hand, there is a 
continued renewal of the solvent, so that, while the most soluble 
constituents are first removed in relatively large quantities, the 
others must yield ultimately to repeated attack and the amount 
finally dissolved depends upon the length of time during which the 
process continues. ‘Thus, to yield comparable results, the labor- 
atory sample would have to be treated repeatedly with fresh 
supplies of solvent. However, this applies more especially to 
chemical weathering where residual clays accumulate to consider- 
able depth and lie a long time. When secular elevation and 
erosion bring fresh rock within the range of solvent action, there 
is a tendency to compensate somewhat for the fresh supplies of 
solvent continually furnished, and thus to bring the conditions more 
nearly into harmony with those of the laboratory experiment. 
But an even closer approximation to the laboratory method is 
obtained if the entire mass of the crust traversed by meteoric 
waters is considered as a unit acted upon by the total body of 
drainage waters which, falling relatively pure upon the land areas, 
immediately begin to exert their solvent power and, ultimately, 
carry to the sea the different constituents of the crust, in varying 
amounts. 
It was from this point of view that the problem was approached, 
when first considered, the composition of the crust and of the 
drainage waters being the basis of calculation. The former was 
afforded by Clarke’s' familiar estimates of the composition of the 
outer shell of the crust and of the sedimentary rocks. Of the aver- 
age composition of the drainage waters, only Sir John Murray’s? 
preliminary estimate was available, and it was because of serious 
doubt as to the accuracy of these figures that publication of the 
results was abandoned. 
While there is still much to be wished for in regard to data 
bearing upon the composition of drainage waters, the last few 
years have afforded sufficient added information in this direction 
to warrant renewed consideration of the problem. 
KF. W. Clarke, Bull. U.S. Geol. Surv., 228, p. 10. 
2 Sir John Murray, Scottish Geog. Mag. (1887), p. 76. 
