RECORDS OF MEETINGS OF 1909 
325 
SECTION OF GEOLOGY AND MINERALOGY. 
December 6, 1909. 
Section met at 8:25 P. M., Vice-President Stevenson presiding. 
The minutes of the last meeting of the Section were read and approved. 
The following programme was then offered: 
C. N. Fenner, Application of the Law of Mass Action to Phenom¬ 
ena of Resorption in Igneous Rocks. 
J. J. Stevenson, The Coal Basin of Commentry in Central France. 
George H. Girty, Tpie Guadalupian Fauna and New Stratigraphic 
Evidence. 
Summary of Papers. 
Mr. Fenner’s paper was intended to show how the law of mass-action 
operates to produce irregularities in the crystallization of a magma. If a 
magma were simply a fusion-solution of certain mineral compounds, which 
were unable to change their relative proportions by inter-reaction under 
changing conditions, then crystallization upon cooling would follow the well- 
known laws of entectiferous solutions with almost absolute exactness. On 
the contrary, however, we know from the principle of mass-action that the 
state of chemical equilibrium in a complex solution is in an unstable condi¬ 
tion and is easily displaced by various influences, among which temperature 
and concentration are chief factors. 
It follows, therefore, that the removal by crystallization of one or more 
compounds from a solution affects the relative concentration of the residual 
material and causes reactions to proceed further in one direction or another. 
The equilibrium may be so far displaced from the condition at which it 
stood when crystallization began that in the later stages the crystals first 
deposited may be attacked and resorbed. 
The removal of material by gaseous emanations upon the extrusion of a 
magma acts in a very similar manner to affect the equilibrium. 
Change of temperature is also an important factor, for this alone will 
cause a change in the conditions of equilibrium within a solution; and a 
mineral which has crystallized from a magma under great pressure at a 
temperature slightly higher than the normal may be resorbed when the 
temperature of crystallization is depressed by relief of pressure, conditions 
of chemical equilibrium having meanwhile shifted. 
