TEE MECHANICS OF GEOLOGIC STRUCTURES 



513 



compression in a direction at right angles to the direction of 

 maximum elongation. 



The pattern of fractures developed in a series of experiments 

 with varying thickness of paraffin coat is uniform in so far that 

 the foregoing described set of 

 fractures are always found. 

 Their relative prominence, how- 

 ever, varies with the thickness 

 and brittleness of the parafhn 

 coat. 



Fractures produced by tor- 

 sional warping. — It is difficult 

 to form any estimate of the 

 importance of this type of def- 

 ormation, but it seems prob- 

 able that torsional warping 

 occurs and, therefore, that it 

 merits consideration as one of 

 the types of earth deformation. 



A warped surface may be 

 considered as having been de- 

 formed in two manners; namely, 

 by the change in area which 

 has engendered tensional or 

 compressional stresses or both 

 and by bending which has occa- 

 sioned stresses characteristic of 

 cross-bending. For purposes of 

 the present analysis it appears 

 best to consider these two 

 phases of deformation inde- 

 pendently. 



Changes in area due to warp- 

 ing. — If a sheet of rubber is 

 held between two clamps, as in 



Fig. 6. — Fractures produced by tor- 

 sional warping. A heavy sheet of rubber 

 is tightly stretched between the two 

 clamps by means of the screw, and coated 

 with parafSn which is made brittle by 

 chilling. Then the lower clamp is rotated 

 by means of the handle at the lower end. 

 This subjects the rubber sheet to tor- 

 sional deformation and develops cracks in 

 the paraffin. (See Fig. 7.) 



Figure 6, and subjected to tor- 

 sion by turning one of the clamps (maintaining a constant distance 

 between the clamps) the effect is to increase the area of the rubber 



