SOLUBILITIES OF CHEMICAL CONSTITUENTS OF ROCKS 107 
That the constituents of rocks and minerals differ widely in their 
solubility is a familiar fact, made apparent by a superficial examina- 
tion of rocks in the field, and emphasized by laboratory experi- 
ments, and the analysis of fresh and weathered rocks and of natural 
waters. As a result of investigations along these lines, it is well 
known that certain oxides, such as silica and alumina, are relatively 
resistant, while others, such as the alkalies and alkaline earths, 
pass somewhat readily into solution. Thus, orthoclase yields its 
alkalies in solution, while the silica and alumina remain in the 
solid state as kaolinite and quartz. 
For our present purpose, this may be expressed simply as due 
to the greater solubility of the alkalies as compared with silica 
and alumina, ignoring the obvious fact that it is, in reality, a 
case of decomposition, not simple solution. It need hardly be said 
that the solubility (as the term is here used) of an oxide varies 
greatly with the compound in which it occurs, but the results of all 
such variations enter into the average relative solubility, as here 
discussed, without demanding any separate treatment. 
Quantitative data in regard to solubilities are generally obtained 
by one of two very different methods. A definite amount of the 
rock or mineral is treated with a definite amount of water, the 
conditions varying with different experiments, and, by analysis 
of the resultant solutions, the relative amounts of the constituents 
dissolved are determined. This method is artificial, but, by a proper 
adjustment of the conditions of the experiment, may yield results 
_ of importance in interpreting natural phenomena.’ The second 
method of acquiring quantitative data is to analyze fresh rocks 
or minerals and the corresponding residual clays or alteration prod- 
ucts and, by a comparison of the results, to deduce the amounts of 
various constituents removed in solution. The first method has, 
thus far, been applied chiefly to minerals, but the second has been 
extensively applied to rocks, and has yielded results of great interest. 
The two methods of investigation are essentially distinct and, 
while they supplement each other, do not yield strictly comparable 
results. In the first, or laboratory, method, a definite amount of 
"Cf. W. P. Headden, “Significance of Silicic Acid in Waters of Mountain Streams,”’ 
Am. Jour. Sci. (4) XVI (1903), 169-184. 
