INTERNAL FRICTION IN ROCKS 637 



4. In the case of the softer rocks — alabaster, steatite, marble, 

 etc. — the deformation is produced by movements due to a slipping 

 within the constituent crystals of the rock on their gliding planes, 

 often accompanied by twinning, the movement in this case being 

 similar to that seen in metals when they are deformed. In the 

 harder rocks the deformation is accompanied by granulation, the 

 texture developed being similar to that found in mylonite. 



5. Each of the softer rocks at least has a well-defined modulus 

 of plasticity. 



6. The "work done" when a rock is deformed by a tangential 

 thrust, within the earth's crust, increases rapidly with the weight 

 of the superincumbent strata, i.e., with its depth below the surface. 



7. The relative ease with which the several rocks will flow under 

 differential pressure is shown in Tables III and IV, which give 

 mathematical expression of the "work done" in deforming standard 

 columns of each rock. 



8. A uniform thrust exerted on'a prism of the earth's crust may 

 deform and fold the upper portion of the mass, while it will be quite 

 insufficient to produce any movement in the lower part of the same 

 mass. 



9. The thrust required to develop deformation, taking no cog- 

 nizance of the influence of heat or the time effect which might result 

 if the pressure were applied with extreme slowness, in the case of 

 marble, and of granite, is shown by the values given in Table V. 



10. To revert to the question propounded by Dr. Gilbert, in 

 order to develop flow in any rock within the earth's crust the rock 

 must be submitted to a differential stress which is greater than that 

 which is required merely to break down its texture and very much 

 greater than that which is sufficient to crush it to pieces under the 

 ordinary conditions which obtain at the surface of the earth. 



