[e873 
XXVIII. 
INFLUENCE OF PRESSURE ON THE SEPARATION OF 
SILICATES IN IGNEOUS ROCKS. By J. JOLY, M.A., 
D. Sc., F.R.S; Hon. Sec. Royal Dublin Society; Professor of 
Geology and Mineralogy in the University of Dublin. 
[Published Octoprr 25, 1900. | 
THE application of the thermo-dynamical formula connecting 
pressure and melting-point with the formation of silicates in 
certain igneous rocks—as more especially Mr. C. HE. Stromeyer has 
lately indicated in his interesting Paper “On the Formation of 
Minerals in Granite ’’!'—calls for some comment. 
It should not, in the first place, be forgotten that the question 
at issue is not simply the transformation of the particular silicate 
from a state of fluidity to a state of solidity. ‘The silicate has 
been built up, molecule by molecule, in the crystalline form in 
the rock. It never at any time existed as a fluid glassy aggregate. 
That it did not is obvious from many considerations, as well as 
from the observed presence of lines of growth and such like 
phenomena. 
Prior to the existence of the crystal the molecules comprising 
it were diffused throughout the magma. Theinfluence of pressure 
here would be exerted in virtue of the total volume-change arising 
in the magma due to the segregation and orderly arrangement of 
the molecules attending crystallization. Is this volume-change 
measured by observation on the transformation of the crystal at 
the temperature of its formation into the melted glass at the same 
temperature? It does not follow. In fact it very certainly is not ; 
for the segregation of the molecules and their consequent with- 
drawal from solution is very probably attended with a consider- 
able volume-change affecting the magma. We might, moreover, 
expect from analogy with known cases that this volume-change 
1 Manchester Literary and Philosophical Society, vol. 44, Part III. 
