194 
IDDINGS. 
rocks of a natural group proves that the differentiation has 
not been in accordance with definite stoichiometric propor¬ 
tions, and that such definite molecules as those designated 
by Rosenbusch as herns cannot he shown to exist in mag¬ 
mas and do not control the final molecular grouping of 
crystallization. It proves the absence of definite silicate 
molecules and the capability of the interchange or shifting 
of the elementary oxide molecules, both in the differentiation 
of magmas and in their crystallization into minerals. This 
negatives the conception of a partial crystallization of mag¬ 
mas and their differentiation by the settling of heavier min¬ 
erals—an idea which is also disproven by evidences of the 
comparatively rapid and relatively recent growth of the 
porphyritical crystals in rocks and the crystallization in situ 
of the minerals of most coarsely granular rocks, and also by 
the uniform distribution of crystals of magnetite and augite 
in such rocks as rhyolites. The differentiation of molten 
magmas must therefore be of a chemico-physical nature. 
Conceptions of the chemical and physical character and 
behavior of solutions are rapidly being developed. Much, 
however, remains to be learned of the true nature of the 
simplest aqueous solution. The complexities of a compound 
solution that exists only at extremely high temperatures 
and experiences the pressures to which rock magmas have 
undoubtedly been subjected may long remain beyond the 
reach of direct investigation. Still the steady advancement 
of experimental physics offers great possibilities in this direc¬ 
tion. Until the establishment of definite knowledge con¬ 
cerning the nature of molten magmas we must proceed 
along the lines of analogy by applying to them such laws 
as may be found applicable to solutions that exist at lower 
temperatures and pressures. 
The most recent theories regarding the nature of solutions 
conceive of a greater or less dissociation of the compounds 
dissolved, or their partial separation into ions, the extent of 
which dissociation depends on the degree of dilution—that 
is, on conditions of relative mass, temperature, and pressure. 
