THE MECHANICS OF GEOLOGIC STRUCTURES 



509 



These are approximately at right angles to the direction of move- 

 ment as is to be expected. They are open cracks perpendicular 

 to the rubber sheet. These results are what an analysis of the 

 mechanics involved would indi- 

 cate. The paraffin, like rock, 

 is less resistant to tensional 

 stresses than to shearing 

 stresses. It fails, therefore, by 

 the development of breaks along 

 planes which are perpendicular 

 to the maximum stress. 



Fractures produced by com- 

 pression.- — The apparatus used 

 for this purpose is the same as 

 the one employed for tension. 

 The compressive force is applied 

 by releasing the screw and 

 allowing the rubber to con- 

 tract. This develops compres- 

 sional stresses in the paraffin 

 coat. The first breaks to ap- 

 pear are small inclined thrust 

 faults striking at right angles 

 to the direction of shortening 

 and dipping approximately 45° 

 either way. (See Figures 2 

 and 3.) These are followed by 

 small number of vertical faults 

 which strike at angles approxi- 

 mately 45° to the direction 

 of shortening (seen near the 

 margin of the rubber sheet in 

 Figure 2) and are due to the 

 fact that lateral relief is afforded at these points by the "spread" 

 of the rubber sheet. There also appear a number of vertical 

 tension joints striking in the direction of shortening and apparently 

 due also to the "spread" of the rubber. 



Fig. 2. — Fractures and faults developed 

 by shortening or compression. This is 

 the same apparatus as shown in Figure i . 

 The heavy sheet of rubber is first tightly 

 stretched by means of the screw, coated 

 with paraffin which is made brittle by 

 chilling, after which the rubber sheet is 

 allowed to contract by means of the 

 screw, thus producing compressional 

 stresses in the paraffin. Figure 3 shows 

 these compression fractures in detail. 



