50 G. F. BECKER — FINITE STRAIN IN ROCKS. 



rupturing stress ; and portions of the rock having dimensions of this order 

 may often properly be regarded as homogeneously stressed. When large 

 masses are similarly strained, gravity may determine in which of several 

 directions, all equally stressed by external pressure, rupture will take 

 place. Cases of such determination 1 have discussed in a formerpaper* 



Effects of direct Pressure. — A direct, uniformly distributee 1 pressure of 

 sufficient intensity, applied to an elastic brittle mass presenting great re- 

 sistance to deformation, would induce fracture. The ruptures would take 

 place along those lines subject to the greatest tangential strain, since 

 these are the directions in which the material would first be strained 

 beyond endurance. These lines would stand at 45° to the line of force 

 if the mass presented infinite resistance to deformation. If this resist- 

 ance is not infinite, they will stand at greater angles to the line of force. 

 The angle which the normal to the direction of rupture makes .with the 

 line of force is called w in the discussion of the strains (see p. 34). 



There will generally be more than one direction of rupture, and in 

 masses the thickness of which in the direction of pressure is considerably 

 smaller than the lateral extension, there will often be four systems of 

 parallel fissures, two systems answering to each of the two equal shears 

 arising from simple pressure. If, however, there is any inequality of re- 

 sistance in the plane perpendicular to the line of pressure, whether this 

 is due to the character of the mass under pressure or to inequalities in 

 the support which this mass receives from its surroundings, the strain 

 ellipsoid will have three unequal axes, and rupture will take place only 

 in the plane of the greatest and the least of these axes. In this very 

 common case the mass will be divided into columns, with angles de- 

 pending upon the strain. "When the mass is large and the pressure is 

 horizontal, gravity opposes the tendency of the vertical axis of the strain 

 ellipsoid to elongate, and rupture will tend to take place by relative 

 motion in horizontal planes, separating the rock into vertical columns. 

 The constraint of surrounding masses may outweigh this tendency. 



Something can be said of the spacing of the fissures thus formed, but 

 this subject can be most conveniently discussed under the head of in- 

 clined pressure. 



If the pressure continues alter rupture has occurred, the Mocks or 

 columns will grind against one another producing slickensideSj and 

 sometimes further ruptures, of which the discussion will also be deferred. 



Many rocks under the action of direct pressures rapidly applied behave 

 approximately as highly elastic brittle masses of greal rigidity, and in 

 these cases the range of the planes of maximum strain is practically nil. 

 Consequently, systems of fissures at sensibly right angles to one another 



♦Bull. Geol. Soc. Am., vol. 2, 1891, p. 62. 



