probably the most abundant, as is plainly indicated by analyses of the gases still remaining in igneous rocks a and by studies of the gases emitted from volcanic vents.6
The
presence of water gas in association with subordinate amounts of other
gases and of certain unusual substances (mineralizers) has been
considered by many observers to be the competent and effective cause in
the development of pegmatitic textures. With this opinion the present
writer is in general accord, though the persuasion is based more
largely on the process of reasoning already outlined than on field
evidence of high-water content or relatively low viscosity in
pegmatite magmas. The field evidence gathered in the study of the
Maine pegmatites is summarized later (p. 45), but must be looked on as
merely suggestive; anything like a complete solution of the problem
will in all probability wait upon synthetic laboratory experiments on
the interaction between gases and rock-forming silicates.
The
small weight of the gaseous and liquid constituents of most igneous
rocks as compared with the total weight of the rock might lead one to
question their competence to notably affect the viscosity of magmas and
to produce large textural variations. In this connection it may not be
out of place to call attention to a possible application of Raoult's
law,0 according to which if various substances are dissolved
in equal amounts of the same solvent in the proportions of their
molecular weights the resulting lowering of the freezing point of the
solution will be the same in each case.d In other words, the
effect produced is a function of the number of molecules concerned and
is not primarily dependent on the nature of the substances introduced.
It follows that a small amount by weight of a substance of low
molecular weight (such as H20, molecular weight 18) will
exert the same depressing influence on the freezing point of the
solution as a much greater weight of a substance of high molecular
weight (such as Fe203, molecular weight 160); and
that given equal weights of the two the substance of lower molecular
weight will exercise much the greater influence. This law has been
found to apply strictly only to very dilute solutions where there is no
chemical action between solvent and dissolved substance. It has been
applied by Vogte to rock magmas, but the wisdom of such
extension to cover widely different and much more complex physical
conditions may well be questioned. It seems not unreasonable, however,
to attribute some general importance to this principle in rock magmas,
to the extent that magmatic constituents of low molecular weight may
exert
o Chamberlin, R. T., The gases in rocks: Pubs. Carnegie Inst. No. 106,1908. This includes a summary of earlier investigations.
6 For a review of the literature on volcanic gases, see Clarke, F. W., The data of geochemistry: Bull. V. 8. Geol. Survey No. 330, 1908, pp. 212-236.
cOstwald, Wilhelm, Outlines of general chemistry, 1895, pp. 136-137.
d Neglecting electrolytic dissociation, which is probably of small importance in rock magmas.
«Vogt, J. H. L., Die Silikatschmelzlosungen, vol. 2, 1904, pp. 128-135,
