THE MECHANICS OF GEOLOGIC STRUCTURES 511 



vertical fracture planes striking at angles of approximately 45° to 

 the direction of compressive force. 



This experiment in terms of earth movements is to be compared 

 with a tangential shortening of an earth mass extending down into 

 the zone of flowage, accompanied by side flowage or spread. This 

 shortening is communicated to the rocks in the zone of fracture, 

 resulting in inclined thrust faults striking normal to the direction 



Fig. 4. — Fractures and faults developed by shear or rotational stress. A heavy 

 sheet of rubber is tightly stretched between the two clamps by means of the screw 

 at the top and coated with a thin coat of parafi&n which is made brittle by chilling. 

 The paraffin-coated rubber sheet is then deformed by means of the screw at the left. 

 The fractures developed by the shearing movement are shown in detail in Figure 5. 



of shortening, vertical shear faults striking at angles of approxi- 

 mately 45° to the direction of shortening and vertical tension joints 

 striking in the direction of shortening. 



Fractures produced by shear or rotational stress. — ^The apparatus 

 used for this purpose is shown in Figure 4. It has one clamp 



