A34 GEORGE F. BECKER 
shears at right angles to one another. The cubical compression 
does not tend to produce rupture or distortion, and each of the 
shears acts independently of the other. Hence in considering 
the effects of direct pressure upon rock, each shear must be con- 
sidered by itself, and the effects combined. Were the resolution 
of forces not what it is, the consideration of pure shear would be 
almost valueless for geological purposes, because the combina- 
tion of two exactly equal loads of opposite signs at exactly 90° 
unaccompanied by other forces must occur in nature only once 
in an infinite number of times. 
If a cube of homogeneous matter is subjected to pure shear- 
ing strain it will be deformed gradually until its elastic lmit is 
reached. With most solids analogous to rocks this amount of 
deformation is very small indeed, so that the shortening of the 
cube at this limit would not exceed one per cent. and might fall 
much short of it. For the purposes of this paper the distortion 
within the elastic limit can be neglected. Just above the elastic 
limit the mass will begin to undergo permanent deformation. So 
far as is known every substance whatever is capable of permanent 
deformation. Were this not true the exceptions to the statement 
would be perfectly elastic bodies. 
The nature of permanent deformation ina pure shear is infer- 
able from what has been stated in preceding paragraphs. It con- 
sists in the motion past one another of circular sections of the 
strain ellipsoid and the motion is such that although the continu- 
ity of the mass is not destroyed, relief from pressure does not 
restore the molecules to their original positions. This irreversi- 
ble movement of particles along the circular planes of the shear 
ellipsoid is the simplest case of what Tresca called the flow of 
solids. It differs fundamentally from the flow of liquids, which 
takes place under corresponding circumstances in a direction 
perpendicular to the line of force, instead of at an angle some- 
what exceeding 45° as is the case with solids. In ordinary solids 
under pure shearing stress flow begins at almost exactly 45° to the 
pressure; as the strain increases this angle increases but it can 
reach go° only when the thickness of the mass is reduced to zero. 
