512 



WARREN J. MEAD 



which may be moved toward or away from the other by means of 

 a screw. The other clamp is mounted in a slide and by means of 

 the second screw may be moved at right angles to the direction 

 of movement of the first clamp. The sheet of rubber is tightly 

 stretched between the two clamps, coated with paraffin, and 

 deformed by means of the screw attached to the sliding clamp. 

 This subjects the rubber sheet and its coat of paraffin to a shearing 



or rotational stress. The re- 

 sulting fractures in the paraf- 

 fin are shown in Figure 5. 

 The direction of movement 

 is indicated by the arrows and 

 the amount of movement by 

 the shape of the parallelogram 

 which was, previous to defor- 

 mation, rectangular. 



The first fractures to ap- 

 pear in any one locality on 

 the rubber sheet are usually 

 tension cracks inclined about 

 45° to the direction of the 

 shearing movement. These 

 are at right angles to the di- 

 rection of maximum elonga- 

 tion and appear as vertical 

 open cracks. They are fol- 

 lowed immediately by two 

 sets of vertical faults with 

 horizontal displacement, one 

 set striking parallel to the direction of movement and the other 

 parallel to the free edges of the rubber sheet. These represent 

 two directions of non-distortion or two shear planes developed 

 by the shearing movement in which direction of relief is in the 

 plane of the paraffin layer. Another set of faults, only two of 

 which are shown in Figure 5, are thrust faults striking approxi- 

 mately at right angles to the tension crack and inclined ap- 

 proximately 45° dipping in either direction. These are due to 



Fig. S- — Fractures produced in paraffin 

 coat on rubber sheet by shearing. The 

 arrows indicate the direction of move- 

 ment and the shape of the figure shows 

 the amount of distortion. 



