e172) Cay SUVEVdGEe voi, 
order of amounts dissolved derived from the study of weathering 
and its solid products, as shown by Merrill,* Van Hise? and others, 
although that method gives no numerical relation, and its generali- 
zations are commonly based chiefly upon igneous rocks. Van 
Hise calls attention to the fact that the different constituents are 
removed in surprisingly similar amounts, but this, of course, con- 
cerns the end result, and takes no account of relative rates of solu- 
tion. As pointed out earlier, the more soluble constituents go first, 
so there is an ever-present tendency toward the relative concentra- 
tion of the constituents of low solubilities, silica and the sesquioxides 
and, to a less degree, potash, in the residual clays. Were there no 
mechanical erosion a certain kind of equilibrium might be reached, 
in which residual clays would have practically uniform compositions 
(varying with climatic conditions) not changing with further solu- 
tion. That there is a tendency to such a condition is emphasized 
by a comparison of the composition of residual clays, showing as 
they do, striking resemblances, with the widely divergent composi- 
tion of rocks. 
It is believed that the above figures for relative solubilities are of 
sufficient interest in themselves to warrant their publication, while 
they touch upon many problems in connection with weathering, 
chemical denudation, metamorphism, etc. Even upon so large a 
theoretical question as the composition and density of continental 
and oceanic areas, as discussed by Chamberlin and Salisbury, 
they have a bearing. 
While it is true that the relative solubilities given above express 
only a most general relation, and cannot be applied to a specific 
case, the simple method of comparison can be used, wherever the 
necessary data are available, with instructive results. From the 
widely varying compositions of streams and of the rocks of their 
drainage basins, it is clear that most divergent results must be 
obtained. It is only as the conditions approach the average for 
the entire land area that any close approximation to the mean 
solubilities may be expected. A few cases are appended by way of 
illustration. 
1G. P. Merrill, Rocks, Rock Weathering and Soils (1906), p. 220. 
2C. R. Van Hise, op. cit. 3 Geology, II, 107. 
