PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 609 
compressed sodium chloride or sodium nitrate in contact with its 
saturated solution to about 200 atmospheres for a period of eight 
days, and found that blocks resembling rock-salt and marble were 
formed in this way. Similarly one can account for the formation 
of rigid sandstones from beds of originally loose sand. 
Application to metamorphic processes——The processes just 
discussed have a very important bearing on rock metamorphism 
wherever stress has been an important factor. The unequal pres- . 
sure increases the activity of the solid components, and fusion or 
solution occurs wherever the local stress reaches the appropriate 
value. In this way larger amounts of material come into local 
solution to be immediately redeposited (at points where the stress 
is smaller) in the form characteristic of those particular conditions, 
and hence not necessarily in the original form; consequently reac- 
tions occur more readily in stressed rocks. The nature and form 
* the new crystals depend therefore upon the relations of stress 
and strain in the mass. The so-called Riecke principle has been 
adduced in this connection; but the whole question has been 
handled in a much more general way by Willard Gibbs,’ from whom 
we quote the following sentences: 
If a solid which is homogeneous in nature and in state of strain is bounded 
by six surfaces perpendicular to the principal axes of stress, the mechanical 
conditions of equilibrium for these surfaces may be satisfied by the contact 
of fluids having the proper pressures, which will in general be different for the 
different pairs of opposite sides, and may be denoted by 9’, p’”’, p’’”’. It will 
then be necessary for equilibrium with respect to the tendency of the solid to 
. dissolve that the potential for the substance of the solid in the fluids shall 
have certain values p’, pw’, w’’’, which are entirely determined by the nature 
and state of the solid. 
From Gibbs’ equations it follows directly that y’ is ‘greater 
than the value of the potential which would be necessary if the 
solid were subjected to the uniform pressure p’”’ (and similarly 
with pies p”, Bt tes ps 
That is, the fluids in equilibrium with the solid are all supersaturated with 
respect to the substance of the solid, except when the solid is in a state of hydro- 
static stress [uniform pressure]; so that if there were present in any one of 
t“Equilibrium of Heterogeneous Substances,’’ Collected Papers, pp. 196-97. 
The question was first treated by James Thomson, Trans. Roy. Soc. Edinburgh, XVI, 
575; Proc. Roy. Soc., XI, 473; Phil. Mag. (4), XXIV, 395. 
