DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 517 



is slowly forced outward, moving along a complicated series of inter- 

 secting planes of shearing, which gives the diamond or leaf-shaped 

 pattern on the surface of the deformed column. 



In some cases where the deformation of the tube has been less 

 symmetrical the upper portion of the column is found to have sheared 

 down over the lower portion, the movement being concentrated along 

 a series of parallel planes forming a sort of distributed fault, the 

 rock, however, still retaining its continuity and being free from 

 fissures (Plate IV, Fig. c). 



When a thin section of a column which has been deformed in alum 

 is examined under the microscope, all the individual grains of calcite 

 are found to exhibit polysynthetic twinning, showing that they have 

 all been more or less deformed, but as before, the chief movement 

 in the rock is seen to have taken place along planes of shearing which 

 traverse the rock and whose course is indicated by little lines of 

 granulated calcite. In order to ascertain the strength of the marble 

 after deformation, two of the deformed columns were tested in com- 

 pression. They crushed at loads of 750 lbs. and 850 lbs. respectively, 

 while a column of the original rock has a crushing load of 4,380 lbs. 

 The deformed marble, therefore, while firm, is much weaker than the 

 original rock. 



2. CUBES, SQUARE PRISMS, AND SPHERES 



In other experiments cubes, square prisms, and spheres of the 

 marble were deformed. In one of these a cube nearly an inch on each 

 side was by compression in three successive copper tubes reduced to a 

 flat cake measuring i .36 inches by i .38 inches and o. 55 inch thick. 

 The maximum load employed was 219,500 lbs., or approximately no 

 tons. 



This deformed cube was still hard and solid and showed no 

 traces of a pulverulent character. It is clear from its form that the 

 movements which it has undergone are identical in general character 

 with those which developed the artichoke structure in the case of the 

 marble columns. On the sides of the cubes two sets of intersecting 

 lines, along which shearing has taken place, are seen. This shearing 

 is most pronounced at the corners of the block which tend to shear 

 down in pieces having approximately the form of the "leaves" of the 



