DEFORMATION OF ROCKS 203 
flow toward the opening and close it. All of the rocks con- 
cerned are everywhere under compressive stress. The material 
at any given moment, when deformed, moves from places of 
great compression to places of less compression. In other words, 
there is always a tendency for the rocks to approach equilibrium 
through the forces applied, or to obey the laws of hydrostatics. 
In order that deformation shall occur, the difference between the 
greatest and the least stresses must equal or surpass the elastic 
limit of the rock in question under the conditions in which it 
exists. In rocks which were bent when so deeply buried that 
no cracks or crevices could form even temporarily, it is probable 
that the material flowed to its new position quietly, without 
shock, under the enormous stress to which it was subjected. 
Even if in the zone of flowage, the relative thickness and 
strength of the members folded will play their part. If the mass 
were exactly homogeneous it would flow in the direction of least 
resistance, like a mass of tallow. But the rock masses are het- 
erogeneous, and the alternating layers of different plasticity may 
retain their individuality, there being no considerable commin- 
gling of the materials of one layer with the materials of others. 
The strong, thick beds will greatly vary the direction of movement 
of the material at a given place, and thus, as explained by Willis, 
develop folds of great length and amplitude. On account of 
their relatively resistant character when bent into anticlines and 
synclines, the anticlines will be able to carry part of the super- 
incumbent load, and thus relieve to some extent the softer beds 
below, which however, as a consequence, promptly flow in the 
direction of relief or least resistance and ever press against the 
confining arch, and thus do their part, which may be the major 
part, of carrying the superincumbent load. In a similar manner 
«The manner of the molecular rearrangement by which flowage is accomplished 
will not here be discussed. In some cases of extreme deformation the granulation or 
even the recrystallization of the rock is complete. In intermediate cases strain effects 
are usually marked. For the present purposes we are concerned with the larger 
deformation of the rock masses. Even were the deformation accomplished wholly by 
minute fractures without crevices, the deformation in mass, would appear to be that 
of a plastic body. 
