430 JOSEPH BARRELL 



with rigidity and strength. Up to a certain degree of strain known 

 as the elastic hmit, elasticity of form in the ideal solid is perfect and 

 is expressed by the law that the change of form, or strain, is directly 

 proportional to the load applied, or stress. This load may be 

 maintained indefinitely and, except for a slight relaxation, the solid 

 shows no further yielding. Upon the removal of the load there 

 is an elastic return to the original form, but the very last stages 

 of the recovery are slow. The elastic nature of the whole earth, 

 in regard to both volume and form, is shown by its capacity to trans- 

 mit the several kinds of earthquake vibrations. The permanence 

 and perfection of the elasticity of form is also impHed in the power 

 of the crust to carry loads up to certain limits for times reaching into 

 geologic periods without exhibiting progressive viscous yielding. 



Beyond the elastic limit, elasticity ceases to be perfect, and a 

 permanent change of form occurs. This relieves part of the stress 

 and reduces the strain to within the elastic limit. The change of 

 form may be by rupture, in which event the strength of the body 

 is destroyed. It may be by plastic flow, in which case the strength 

 may be increased or decreased. Wrought iron, for example, 

 becomes somewhat stronger as a result of forging. Granite on 

 being mashed into gneiss becomes somewhat weaker because of 

 the development of weaker minerals, especially the micas. In the 

 crust of the earth, except for the outer few miles, flowage takes 

 place without probably much change in the mineral composition 

 and consequently in the strength of the rock. Deformation will 

 continue as long as the stress is maintained well above the elastic 

 limit, but upon the cessation of movement there may still remain 

 residual stresses up to the elastic limit. If the residual stresses 

 over broad areas are small, it may be because the development of 

 weaker structures, such as folds or zones of igneous injection, has 

 eased the strains. 



Failure by flow brings in the distinction between viscosity and 

 plasticity. These are often used, even by physical geologists, 

 as merely synonymous terms, but there is a real distinction which 

 should be noted. Fluids are viscous to a small or large degree and 

 can have no elasticity of form. Viscous flow must, however, over- 

 come internal friction and requires time for its accomplishment. 



