268 BECKER 



These isomorphous or pseudo-isomorphous substances rup- 

 ture in 2 ways, both of which may often be illustrated in the 

 same experiment. One species of fracture takes place by ten- 

 sion, and is usually characterized by sharp curvatures and un- 

 ■even surfaces ; the mass is torn asunder. The other method of 

 fracture is by "shearing motions," due to pressure; the mass 

 is ctit to pieces by surfaces which are often, and in fact char- 

 .acteristically, flat and smooth. 



Persistent joints and systems of joints are due to pressures 

 ■while the partings between columnar basalts and the very sim- 

 ilar cracks in drying mud arise from tension. In mining dis- 

 tricts tension cracks often appear as a subordinate phenomenon 

 where faulting has forced apart slaty walls, leaving splinters 

 attached to both sides of a fissure which itself arose from pres- 

 sure, and I have even seen similar occurrences along the crev- 

 asses of a glacier. So, too, when a cylinder of relatively mild 

 steel is crushed, the bulging edge of the mass may show merid- 

 ional tension cracks due to the increase of the equatorial periph- 

 •er}', even when the interior displays diagonal fracture. The 

 behavior of cylinders, however, has some peculiarities which 

 will be mentioned presently. 



Rocks are often ruptured without much preliminary^ deforma- 

 tion, and it is easiest to begin with the hypothesis that the de- 

 formation is negligibly small. The effect of larger deforma- 

 tion can be traced after the principal characteristics of rupture 

 ihave been examined. It is also convenient to consider first of 

 :all a cubical or at least a rectangular mass. 



Suppose then that a cube of rock (shown in Fig. i, PI. XII) 

 is subjected to a perpendicular and evenl}^ distributed force 

 acting on its upper and under surfaces while the face A and that 

 opposite to it are supported in such a way as to obviate rupture. 

 Then the effect of force will be to produce ruptures along planes 

 perpendicular to A and inclined in opposite directions at an 

 angle of 45° to the line of force. Two systems of joints will 

 result forming angles of 90° to one another on A or on sections 

 parallel to this face. On the face of the cube marked B and 

 that opposite to />', the traces of these joint planes will be hori- 

 zontal straij^ht lines, while on the surfaces on which the forces 



