EFFECTS OF STRAIN DISCRIMINATED. 65 



Various Results of Strain. — In the foregoing pages a theory of slow rup- 

 ture lias been presented, which will be supplemented a little later by 

 considering the possible effect of vibration in those cases in which the 

 rupture is sudden. Observation seems«to indicate that many rocks have 

 been strained to the breaking point so gradually that the theory developed 

 is applicable.* 



In applying the results reached to the elucidation of geological phe- 

 nomena the physical character of the rock must be carefully considered. 

 Some rocks when strained with moderate rapidity approach in behavior 

 the ideal, elastic, brittle, non-viscous solid. h\ such cases an inclined 

 pressure will produce two systems of cracks sjich as those illustrated in 

 figures 10 and 11. If the mass is held in the strained state, so that the 

 fragments have no opportunity to recoil, the direction of the force may 

 then be inferred approximately by mere inspection. The plane in which 

 it lies will be perpendicular to th'o systems of fissures. Its direction will 

 intersect the obtuse angle made by the fissures, arid it will make a smaller 

 angle with the short side of the parallelogram of cracks bounding a 

 column of the rock than with the long side. The direction of the force 

 (•.■in be calculated exactly from the lengths of the sides of the parallelo- 

 gram and the angle between them, if Poisson's hypothesis is assumed. 



If the rock is viscous but not plastic (or if it is strained under such 

 conditions as to bring the viscosity into play, but not to keep the rock 

 for a considerable time in a state of strain exceeding the elastic limit and 

 falling short of the ultimate strength), the effect of the viscosity on the 

 long sides of the parallelogram will be far greater than on the short sides, 

 because of the difference in range of the two planes of maximum tangen- 

 tial strain. He.nce fissures will form only in the directions of the short 

 sides of the parallelogram and the rock will be divided into sheets. 



If the conditions are such as to develop both the viscosity and the 

 plasticity of the rock-mass, flow will tend to take place parallel to the 

 short side of the parallelogram because of the inferior viscous resistance. 

 If the plasticity is sufficiently great, the strain will not manifest itself as 

 rupture in this direction, but merely as plastic deformation. If the plas- 

 ticity is not sufficient to prevent all rupture, it will at Least diminish the 

 amount of rupture needful to relieve the strain, and there will be mingled 

 effects of deformation and rupture. 



These mingled effects might consist either in a wider spacing of this 

 set of fissures or in the distribution of short cracks through the mass. 

 Of these the latter seems the more Drobable. The area corresponding to 



♦ Striking instances of the rupture of casi iron blocks are depicted in the frontispiece of Tod- 

 hunter's History of Elasticity. In a general way they accord with the theory developed in the 

 text; but the blocks employed were too slender to give the full syst :m of fissures demanded by 

 the theory for slabs of moderate thickness and greai area. 



