PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 611 
lel to one another; their arrangement thus corresponds exactly to 
flow cleavage. 
The anisotropic character of minerals (also with respect to their 
solubility relations) introduces a complication, without, however, 
affecting the general principle involved. A definite arrangement 
of minerals with respect to a single vectorial property is often 
found; such an arrangement can of course only be ascribed to the 
vectorial properties themselves, whether they be differences in 
solubility or in cohesion. 
In metamorphic rocks exhibiting flow cleavage the flakes of 
mica lie in such a way that the smallest growth was perpendicular 
to the base, the greatest growth in the basal (and cleavage) plane. 
We must remember that the immediate influence of stress and 
strain is made evident in an unequal development in different 
directions, although the general principle alone gives us no informa- 
tion as to the particular vectorial direction which is parallel to the 
smallest development. But when we take into account the ani- 
sotropy exhibited by the original tiny particles, we must include 
as one of the factors the orientation of the original particles with 
respect to certain directions within the crystal; those directions, 
namely, parallel to the plane (gliding plane) in which displacement 
of the crystal particles is most readily effected, and perpendicular 
to the plane (cleavage plane) in which the crystal most readily 
splits. When the rates of growth or the solubility relations are 
anisotropic, specific effects analogous to the above will be produced; 
but of this influence little that is definite can at present be said 
except that it certainly is appreciable. 
From this it follows that the relicts will tend to assume the 
form of the strain-ellipsoid (if we neglect for the moment the com- 
plications introduced by anisotropy). And indeed it is well known 
that the quartz relicts in rocks originally conglomerate are more 
or less oval in form. Further, the recrystallization will tend to 
take place in such a way that the strain is a minimum. This is 
the basic principle underlying the phenomenon of flow cleavage; 
for it corresponds to minimum expenditure of work in producing 
the new formation, or in other words it represents the most com- _ 
plete adaptation of the system to the strains acting upon it.' The 
™P. Niggli, Beitrdge geol. Karte Schweiz, N.F., XXXVI (1912), 46. 
