106 CHS MVE Ey iixe 
and, though still defective in certain respects, they afford the needed 
basis for a preliminary discussion of the problem. 
At the outset, it should be made clear in what sense the term 
‘‘relative solubilities” is used. Probably, all but the simplest miner- 
als, when acted upon by water, do not merely dissolve, in the strict 
physical sense of the word, but decompose,' so that the constituents 
appear in the solution in quite different proportions from those in 
which they are present in the mineral. As rocks are mixtures of 
minerals, it is obvious that, when they are acted upon by water, 
there must be a very complex series of decompositions and inter- 
actions. With the details of these operations, the present inquiry 
is not concerned, but merely with their final results as represented 
by the materials in solution, derived from the rocks. For this 
purpose, the latter are regarded as composed, not of minerals, but 
of oxides, as ordinarily expressed in the results of chemical analysis. 
Such a conception of the rocks requires, of course, no apology, as 
it is necessary, and universally used, in dealing with them from 
the chemical point of view. 
Thus, by the term relative solubilities, as here used, is meant the 
relative amounts of the elements, as oxides, abstracted from the 
rocks of the crust, by drainage waters, as compared with the 
amounts of the same oxides in the rocks. Obviously, this use of 
the word solubility is quite different from the strict sense, where a 
solution acts upon a homogeneous substance, without decomposi- 
tion, but no other term expresses the idea as well, and such a usage 
is so familiar to geologists as to justify it in the present connection. 
A similar plea may be made for considering the oxides, the “chemical 
constituents” of rocks, as distinguished from the actual minerals. 
As before stated, these oxides are continually made the chemical 
units in the discussions of rocks, and this well-established custom 
necessitates, in the present discussion, the expression of water 
analyses in oxides rather than ions, as is the current custom. The 
use of ions for both classes of analyses would lead to the same 
results, but would, of course, be highly artificial, as well as 
unfamiliar, in the case of rocks. 
™Cf. A. S. Cushman, and P. Hubbard, The Decomposition of the Feldspars, U.S. 
Department of Agriculture, Office of Public Roads, Bull. No. 28, p. 10, 1907. 
