5IO WARREN J. MEAD 



An analysis of the mechanics involved leads to conclusions in 

 accordance with the foregoing experimental results. Under simple 

 compression, fracture takes place by breaks which develop in the 

 planes of maximum shear/ Therefore the paraffin should fracture 

 along planes inclined at approximately 45° to the direction of the 

 compressive force. The development of these inclined shear 

 fractures requires actual displacement on the plane of fracture. 



Fig. 3. — -Fractures and thrust faults produced by shortening or compression in 

 the direction of the arrows. (See Fig. 2.) The white bands are fractures and thrust 

 faults in the parafi&n which strike at right angles to the direction of shortening and 

 dip at angles of approximately 45° in either direction. 



This movement has a component parallel to the compressive force 

 and also one at right angles to this force. Evidently, therefore, 

 fractures can develop only in such an attitude as permits this 

 movement to occur. In the central part of the rubber sheet the 

 direction of easiest relief is upward or away from the surface of 

 the rubber and therefore we expect inclined fracture planes strik- 

 ing at right angles to the direction of compression. Near the 

 margin of the rubber sheet lateral relief is afforded, and we find 



^ C. K. Leith, Structural Geology, p. 16. 



