432 GHOTRGE Ts BE CER: 
planes are also at the same distance apart after strain as before, 
for otherwise the volume of the mass must have undergone 
alteration which would be inconsistent with the definition of pure 
shear. Since these planes have the same shape, dimensions and 
distance apart after strain as before it, there is but one change 
which they can possibly have undergone, viz., a gliding move- 
ment past one another. One may regard the entire ellipsoid as 
intersected by planes parallel to the circular sections and very 
close together. Consequently also. the process involved in a 
shear consists solely in the sliding past one another of the thin 
plates bounded by such sections.t. A convenient model illustrat- 
ing the nature of shear is a bit of wire netting. Ifa piece of 
such netting is pulled diagonally to the mesh, each of the two 
~ systems of interwoven wires is distorted much like the traces of 
the corresponding system of circular sections in the shear ellipse. 
The circular sections must necessarily be planes on which 
THe MOnees) ake purely tangential; for if the forces had any nor- 
mal component whatever distortion would ensue. Now it is easy 
to show that in any shear, however great, the load (or the force 
per unit area multipled by the area) is exactly the same for 
every central section passing through the mean axis.? In gen- 
eral this load is inclined, so that it has both a component perpen- 
dicular to the given section and also a second component tan- 
gential to it. On the two axial sections of the ellipsoid, (2. ¢., 
the central sections perpendicular to the greatest and least axes) 
these loads are exactly normal to the surfaces. On the two cir- 
cular sections the loads are exactly tangential. Since the total 
load is the same in all cases, the tangential load is evidently a 
maximum when the load is wholly tangential or when the section 
considered is the circular section. 
It is possible to make geological applications of the theory 
of pure shear stated above provided that the reader will take for 
‘During the actual process of straining from a sphere to a given shear, even those 
material planes which are undisturbed at the end of the process undergo distortions, 
but these deformations are equal and opposite. 
2 This was first shown, I believe, in the paper already referred to, p. 37. I have 
given a neater proof in Amer. Jour. of Sci., Vol. XLVL., 1893, p. 339. 
