COMPLEXITY OF RUPTURES IN ROCKS. 601 



in the direction of compression, the inclinations of rupture are inclined at 

 angles less than 45° to the direction of greatest compressive stress. 



The actual direction of fracture depends not only upon the direction 

 and nature of the stresses, but upon the character of the rocks. The 

 directions of rupture are greatly influenced by the variable strength of the 

 rocks, by the rock structures and textures, aud by the difference in size and 

 strength of the constituent particles. Because of these varying factors 

 force is transmitted with unequal efficiency in different directions. Conse- 

 quently the direction of the maximum stress is not the same throughout a 

 rock mass, but varies from place to place. This variabilit}^ both in the 

 nature of the rock and in the direction of the stresses gives very great 

 irregularity of rupturing. Because of these variable factors no comprehen- 

 sive statement can now be made as to the directions of ruptures in rocks; 

 but certain special conclusions are evident. Where the rocks are hetero- 

 geneous the ruptures are likely to take place to a greater extent in the 

 weaker rocks. Where there are contacts' of strong and weak rocks ruptures 

 are likely to take place at the contact planes, for these are planes of weak- 

 ness. Where there are regular structures, such as bedding, gneissositv, 

 schistosity, cleavage, the ruptures are likely to follow these planes of weak- 

 ness. Where in a rock the particles have variable strength, weaker particles 

 suffer more than the stronger ones. In the rupture of mineral particles 

 fractures are likely to follow cleavage planes, since these are planes of 

 weakness. All these factors result in giving ruptures which vary greatly 

 from positions at right angles to tensile stresses and from 45° to the 

 maximum compressive stresses. 



It follows from the foregoing that the determination of the precise 

 direction and nature of the forces which produced rupture in any individual 

 rock is a most difficult matter. 



After raptures are produced differential movements may result in the 

 rubbing of the parts adjacent to the fractures against one another. At such 

 places the individual mineral particles may be granulated. However, this 

 fracturing of the mineral particles is largely confined to thin layers along 

 the walls of the openings. This fracturing of the particles is in many 

 respects like the fracturing of mineral particles in the zone of anamorphism. 

 (See pp. 673-675.) The matter is not further considered here. 



From the foregoing it follows that deformation accomplished by frac- 

 ture in the belt of cementation results in breaking- up the rocks into great 



