74 
HAYFORD. 
Finally, it must be borne in mind continuously by those 
studying the earth that there is no fixed and necessary rela- 
tion between rigidity (or stiffness) and strength, and that 
a given portion of material may behave in an elastic manner 
under one set of suddenly applied stresses, while at the same 
instant it is yielding slowly in a non-elastic manner under 
other stresses acting for long periods without change of 
direction. 
