454 STUDIES FOR STUDENTS 
age cleavage, or cleavage proper, as the term is here used, the 
thrust may be from one or more than one direction. It may 
vary in force both horizontally and vertically. In any case 
there is flow of the rock-mass in the direction of least resistance. 
If the force be applied so that there is uniform shortening in 
one direction, as in the case of a rigid piston, the elongation is 
at right angles to the direction of thrust, or in the normal planes. 
This may be called pure shortening (Figs. 1 and 2). By pure 
shortening is meant the particular kind of non-rotational distor- 
tion illustrated. The volume remains unchanged, the shortening 
in one direction being compensated by equivalent elongation at 
right angles to this. Jn this kind of deformation, while there is 
no differential movement or shearing in the normal planes, shear- 
ing does occur along all of the intersecting diagonal planes. It 
makes no difference whether the movement is wholly from one 
end of the mass or in part from both ends. But when the lateral 
force varies greatly at varying depths the deformation may vary 
from pure horizontal shortening to a nearly horizontal differential 
movement combined with shortening (Figs. 5 and 6). Such 
inclined differential movement is dependent both upon the vary- 
ing force of thrust in passing from higher to lower horizons and 
upon increased friction with greater depth, due to gravity, which 
acts as anopposing force. Inthe case last illustrated the deforma- 
tion isa simple shear.‘ The deformation here called pure short- 
ening is defined by Becker? as a shear, and a shear, as just 
defined, is called by him scission. 
Ordinarily, at any place in the rock-mass the three principal 
compressive stresses are unequal. If the differential stress 
surpasses the elastic limit of the rock it produces shortening in 
the direction of greatest stress, elongation in the direction of 
least stress, and shortening or elongation in the direction of mean 
*Text-book of the Principles of Physics, by Alfred Daniell, 3d edition, 1894, p. 
78. Macmillan & Co., London and New York. Elementary Text-book of Physics, 
Anthony and Brackett, 4th ed., 1888, pp. 113-114. Treatise on Natural Philosophy, 
Thompson and Tait, new ed., 1890, Part I, pp. 123, 124. 
2 Finite Homogeneous Strain, Flow, and Rupture of Rocks, by G. F. Becker, Bull. 
Geol. Soc. Am., Vol. IV, 1891, pp. 22-25. 
