68 
OROGENY AND EARTH’S ROTATION 
of rubber, the release of the latter not only swells the lower layers 
of the clay, but contorts, folds and fractures the upper and harder 
layers. 
Bathymetrically, the horizon at which, under stress, rock-masses 
cease to break and begin to undergo molar transformation, is not a 
definite theoretical depth as usually inferred — a distance from 
the surface where the strongest rocks commence to be crushed, or 
a level five to six kilometers down. The controlling factor of 
rock-flow appears to be not so much any absolute load of super¬ 
incumbent materials, as it is the lithological, or textural, nature 
of a given rock prism. Although the crushing strength of the 
toughest granite is a column ten kilometers, or so, in height, per¬ 
fect slipping of component particles over one another, which in all 
essential respects is true rock-flow, occurs in such formations as 
shales within a few hundreds of meters of the earth’s surface. 
As is well known, the property by which clays or shales slip with 
greater ease in some directions than in others is due primarily to 
the micaceous, or lamellar, character of some of the components. 
Whether the shale beds be thick or thin the superior proneness t6 
glide in a direction parallel to the original bedding planes persists. 
This ready movement in shales is essentially identical with the 
“flow” of crystalline rocks at great depths and under great pres¬ 
sures. It is strictly intermolar motion. It takes place not alone 
in thick beds of shale but in thin partings between brittle forma¬ 
tions. It is apparent in the thinnest films separating limestones 
and sandstone beds. The evidences of intermolar movement are 
particularly noticeable in sections in which there are great succes¬ 
sions of thin limestones and shales. The coal measures of Kansas, 
Iowa and Maryland afford illuminating examples. 
The distinction usually drawn between the zones of rock-flow 
and rock-fracture appears, therefore, to be fanciful rather than 
real. There is a relative rather than an absolute difference. The 
time element is more important than either load-factor or molar 
constancy. True deformation, or bending, of slabs composed of 
the most brittle, crystalline and rigid of rocks takes place at the 
surface of the earth with no load whatever and with the only 
stress imposed by their own weight. Thin grave-slabs of marble 
supported at the ends are known to sag several inches in the 
middle in the course of a few years. In examples of this kind 
