SCHISTOSITY AND SLATY CLEAVAGE 439 
is common in nature.! When this strain is pushed to the point 
of rupture it leads to a solitary fault. It is nearly the strain pro- 
duced when a mass is being cut with shears, and it can be illus- 
trated with a pack of cards. Scission is that strain in which the 
distance of any particle from two rectangular planes of reference 
is unchanged, while its distance from a third plane perpendicular 
to the two others is simply proportional to its distance from one 
of those others. Thus if 2, y, 2 are the initial co6rdinates, 
4 Vy 2, the final codérdinates of a point, and 6a constant, # = 
4.,), =. 2 = 2, O represents a scission. This is illustrated in 
Fig. 4, where 6 = tan§. This strain is not attended by a change 
of volume. 
Instead of discussing the nature of rotational strains in general, 
I shall simply show how to trace a feature of scission which is in 
fact the effect of rotation. In scission, as in pure shear, the 
deformation is effected by the gliding of planes answering to the 
circular sections of the strain ellipsoid; but the range of these 
two sets of planes through the mass subjected to strain is not the 
same on each side of the line of pressure. In pure shear the two 
angles A and + are equal. In scission this is not the case; on 
the contrary 7 becomes zero and & gains what 7 has lost. When 
the mass has a low limit of elasticity the initial lines of flow will 
may either define the couple by the three angles which its pole makes with the princi- 
pal normal stresses or one may resolve it into three couples with poles coincident with 
the principal stresses. 
The forces at a point are thus defined by six independent quantities, and thes- 
correspond to six independent resistances which are similar in character to the extere 
nal forces but not in general similarly oriented. The work done against elastic resist- 
ance during a small strain is a homogeneous quadratic function of these six quantities. 
Such a function contains thirty-six coefficients which reduce to twenty-one by identities. 
These are the “twenty-one elastic coefficients” of eolotropic matter which are very 
famous. They have even been celebrated in verse! If the elastic forces between two 
molecules are reducible to asingle force acting between their centers of mass (a theory 
incorrectly ascribed to Boscovitch) the twenty-one coefficients reduce to fifteen; but 
this theory is not borne out in general by experiment though some substances, such as 
glass, closely fulfil its conditions. 
Amorphous substances have two elastic constants and regular minerals have only 
three, but triclinic crystals boast the full number of twenty-one. 
« Purely tangential force would expand itself close to the surface to which it was 
applied. Slickensides may be regarded as due to approximately pure scission. 
