OROGENY AND EARTH’S ROTATION 
71 
circumference at a considerable angle. Although the effect is not 
yet demonstrated directly by experiment, it is probable that the 
shear-plane is a truly warped surface, deflecting under gravity con¬ 
trol more and more outwardly and radially as the face of the 
spheroid is approached. 
The geological application of these principles appears to find 
abundant exemplification. The position and frequency of thrust 
lines in the older rocks suggest that the fact is perhaps due mainly 
to the once deep-seated character of these terranes. If, as already 
surmised, some of the major faults displayed by the Great Basin 
structures are in reality shear ruptures instead of gravity faults 
our conceptions of the ancient Basin Range tectonics undergo 
complete change. 
A rotating spheroid of moderate rigidity does not, as is readily 
understood, have its geometric radius coincident with its radial 
line of molar equilibrium, or radius of repose from stress. The 
first is a straight line. In marked contrast the second is a section 
of a parabolic curve the focal coefficient of which varies with the 
rate of revolution. Both the path of this equilibria! curve and the 
variant values of the stress produced by given rates of revolution 
are capable of exact mathematical expression. The resulting 
equation is easily made general. 
For obvious reasons the miniature spheroid of the laboratory 
acts as a perfectly homeogeneous body. The nearest approach 
to nature is a hollow rubber ball, filled with viscous fluid, or soft 
wax. Applying by analogy these conditions to the earth the 
problem at once loses much of its simplicity. Various complica¬ 
tions set in. The so-called zones of rock-flow and rock-fracture 
have to be treated separately. Behavior of the two zones is neces¬ 
sarily very different. The former acts as a homeogeneous body 
under hydrostatic pressure. The latter partakes of the character¬ 
istics of a heterogeneous mass. Cummulative stress is relieved 
through flexure, rupture, or shearing. This gives rise to all those 
tectonic phenomena which are commonly accounted for on the 
hypothesis of a shrinking nucleus. Tangential compression is thus 
initiated without recourse to the assumption of a cooling globe. 
As one of the less familiar types of crustal displacement the 
thrust-fault is not subject to the same critical analysis that the 
simple gravity-fault enjoys. So long as the reversed fault is look- 
