600 JOHN JOHNSTON AND PAUL NIGGLI 
is not re-established when the compression ceases. The effect of 
differential compression may be resolved into two parts: that due 
to a (smaller) uniform pressure and that due to a shearing stress, 
the latter being the preponderating effect. The mere fact that 
“flow”? or deformation of the compressed material occurred is 
sufficient evidence that unequal stresses were set up; in other terms, 
that the compression was not uniform. In discussing this question 
it will be convenient to treat first the effects of stress on a solid 
phase alone before passing on to the consideration of systems solid- 
liquid or of those in which we may imagine the appearance of a 
liquid phase to be one of the results of stressing the solid phase; 
the latter is much more fundamentally important in relation to 
metamorphic processes than the former. 
The effects of stress on the solid phase alone.—The effects of stress 
on solids are what we are accustomed to call mechanical; they are 
subsidiary, rather than essential, in metamorphic processes. For 
this reason, and because the general subject has been discussed 
by Willard Gibbs, while its geological applications form the sub- 
ject of a number of well-known papers,’ the effects of stress on solids 
will be treated very briefly here. 
Any directed stress acting on a particle may be resolved into 
three stresses, each acting on an element of surface in one of the 
three principal planes. These three stresses are represented both 
in direction and intensity by the three principal axes of an ellipsoid. 
The stress produces in the particle a strain, which will tend to change 
the shape of the particle, or its size, or both together. At any 
point there will be three mutually perpendicular directions in which 
the displacement is a maximum or minimum and about which 
these displacements will be symmetrically arranged as an ellipsoid 
is about its three axes. A homogeneous isotropic sphere exposed 
to the action of a stress will assume the form of the so-called strain- 
™C. R. Van Hise, ‘“‘Metamorphism of Rock and Rock Flowage,” Bull. Geol. 
Soc. Am., IX (1898), 269; G. F. Becker, ‘‘ Finite Homogeneous Strain Flow and Rup- 
ture of Rocks,” Bull. Geol. Soc. Am., IV (1893), 13; ‘‘Schistosity and Slaty Cleavage,”’ 
Jour. Geol., TV (1896), 429; ‘‘Experiments on Schistosity and Slaty Cleavage,” Bull. 
U.S. Geol. Survey 241 (1904); C. K. Leith, ‘Rock Cleavage,” Bull. U.S. Geol. Survey 
239 (1903); L. M. Hoskin in 16th Annual Report, U.S.G.S., Part I, 1896; P. Nigeli, 
Beitrdge geol. Karte der Schweiz, N.F., XXXVI (1912). 
