SOLUBILITIES OF CHEMICAL CONSTITUENTS OF ROCKS 113 
rocks, a much lower, and essentially abnormal relative solubility 
must be obtained. While there is a tendency in the same direction 
for MgO, it is too slight to account for the lower figure. The 
fairly good agreement of the other constituents is an argument in 
favor of the essential reliability of the data used for the calculation, 
and gives added confidence in the relative solubilities of columns 
III and IV. 
It is evident that,-on the basis of solubility, the constituents 
fall into four distinct groups; lime and soda, magnesia and potassa, 
and silica and sesquioxides, although potassa might be classed 
with the last two, making only three groups. 
It is unfortunate that alumina and iron oxide cannot be esti- 
mated separately, particularly with reference to the history of the 
sedimentary iron ores, but in the analyses of river waters used as 
the basis of calculation, they are determined together and, thus, 
cannot be separated here. Furthermore, these constituents occur 
in small quantity in waters, are difficult to determine accurately 
and, therefore, even when taken together, their relative solubility 
is open to question, although its order of magnitude is clearly fixed. 
The figures of columns III and IV, then, in so far as they are 
accurate, express a most general relation existing between the 
chief chemical constituents of rocks with reference to their behavior 
under the solvent action of drainage waters. Based, as they are, 
upon analyses of the present surface rocks and existing rivers, 
they hold good only for existing conditions: petrologic, topographic, 
and climatic. A marked change of any of these conditions would 
produce a corresponding change in the relative solubilities, and 
there can be no doubt that many such changes have occurred. 
Doubtless, on the average, the divergence from existing values 
was greatest in the most remote geological times, and this must be 
taken into account in all considerations of chemical denudation in 
earlier periods. However, it is obvious that the intervals between 
the solubilities of the four groups are of such magnitude as to allow 
of considerable range, without changing the order, so much so as to 
make it not unlikely that the order has been the same throughout 
geological time. 
This order, it may be noted, is in general agreement with the 
