PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 601 
ellipsoid, the position of which, and thus the state of strain, at any 
point is completely determined. 
The strain- and stress-ellipsoids are parallel to one another only 
in isotropic media, and then only when the stress acts continuously 
in a definite direction. The presence of particles of unequal hardness 
—in other words, heterogeneity of the material—results obviously 
in the cessation of this parallelism. The position of the strain- 
ellipsoid is influenced by the exact distribution of the particles of 
unequal hardness; and, on the other hand, the position of these 
fine layers—in so far as it can change at all—is determined by the 
direction of the strain-ellipsoid (tectonic displacements). The 
orientation of newly formed mineral grains is determined in general 
by the relations of stress and strain, the most thorough adaptation 
being attained when flow cleavage takes place. In the crystalline 
schists formed under stress which show heteroblastic structure the 
orientation of the porphyroblasts is indefinite, which is presumably 
due to their rate of formation. That their orientation does not 
correspond to the conditions of strain is evident from the fact 
that subsequent rotational movements have sometimes occurred," 
accompanied by cataclastic phenomena. Cataclastic effects pre- 
dominate in the so-called mylonitized rocks, which are found prin- 
cipally where thrusting has taken place. 
The optical properties of crystals are influenced by stress. 
Isotropic minerals may become biaxial, if the direction of the prin- 
cipal stress does not correspond with that of the axes of symmetry. 
Uniaxial crystals become optically biaxial when the direction of 
the stress is other than that of the optic axis of the crystal; for 
instance, in crystalline schists quartz occurs frequently as optically 
biaxial crystals, which at the same time show elliptical, in place 
of circular, polarization, and undulatory instead of definite extinc- 
tion. Application of stress changes in general the position of the 
principal axes of biaxial crystals; when the stress-ellipsoid is paral- 
lel to the ellipsoid derived from the crystalline axes there will be 
a change only in the refractive indices and in the optic axial angle. 
Furthermore, stress may to some extent produce gliding or 
translational displacements in the individual mineral grains. As 
1P, Niggli, Beztrage geol. Karte Schweiz, XXXVI (1912), 61. 
