430 CERORGE MF. BE CREE 
On the other hand experience must be appealed to in the pres- 
ent state of knowledge to decide whether the surfaces of struc- 
ture will show diminished or increased resistance to splitting. 
This theory differs very radically from the older one accord- 
ing to which cleavage occurs only in heterogeneous masses and 
is normal to the causative force. It is certainly important for 
geologists to decide between the two, for the effect of geolog- 
ical forces is chiefly manifested in structures of the kind under 
discussion. In my opinion joints, slickensides, faults, systems 
of veins, schists and slates are all closely allied manifestations 
of force and the true theory of one of them must explain them 
all. These structures constitute the alphabet of the dynamic 
record. Upheaval and subsidence will never be elucidated until 
the history of mountain building can be correctly spelled out. 
The term ‘strain’ as applied to a solid body signifies a 
change in shape or size such as would result from the action of 
external forces.‘. Acquaintance with two strains only is requisite 
for the purposes of this paper. One of these will be called 
“pure shear” or simply ‘‘shear,’’ and the other will be called 
“scission.” 
Pure shear is the simplest conceivable strain. It involves 
only a change of shape, and this change takes place only in two 
dimensions. Nevertheless it presents interesting and important 
properties. Ifa cube of any solid be subjected to a uniformly 
distributed load, acting normally as a pressure on two opposite 
faces, and is at the same time affected by an equal load acting 
as a normal tension on two opposite faces at right angles to the 
first pair, as shown in Fig. 1, it will be elongated in one direc- 
tion and contracted in the other. Such a distribution of forces 
will not alter the volume; there will be no change of length or 
direction in lines perpendicular to the plane of the forces; and 
lines originally parallel to either pair of forces will remain 
* Stress is force measured per unit area; or in the case of “bodily” forces such 
as gravity per unit volume. In the mechanics of elastic bodies stresses are either the 
external forces which cause strain, or the equal and opposite elastic resistances which 
the external forces excite in the strained mass. 
