DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 515 



in a copper tube, also of the same dimensions as that employed in the 

 experiments just described, but. with sulphur as an embedding 

 material. The deformation required a load of 37,500 lbs. (17,000 

 kilos). When the sulphur was melted away from the marble, the 

 deformed column was found to be hard and solid, and to have been 

 reduced in height from 1.561 inches (39.65 mm.) to i . 145 inches 

 (29 . 09 mm.) , a shortening of about 27 per cent. Its shape was striking, 

 for one half of the column had sheared down over the other half, the 

 plane of shearing making an angle of 36° with the vertical, which 

 was the direction of pressure, and an angle of 54" with the horizontal, 

 which is the same angle as that observed in the case of the lines 

 traversing the surface of the marble when deformed in parafhne. 

 This shearing movement did not take place on a single plane, but on 

 a series of planes parallel to one another, or nearly so, and close 

 together, giving rise to exactly the same structure as that seen in the 

 "sheeted veins" or "shear strips" of many mining districts, as for 

 instance at Cripple Creek. It is an excellent example of a distributed 

 fault. The second series of lines seen in the paraffine experiments 

 are here faintly indicated in a few places. There are no signs of 

 rupture to be seen in the deformed column. The surface is nearly 

 smooth, its only unevennesses being due to slight projections along 

 the line of some of the planes constituting the distributed fault. 



A series of thin sections was prepared, passing through the 

 deformed column in a vertical direction. Under the microscope it is 

 seen that there has been a slight movement throughout the rock, as 

 indicated by the presence of a fine polysynthetic twinning in almost 

 every calcite grain. This, however, has not been sufficient noticeably 

 to flatten the grains in any direction. The chief movement is along 

 the planes of shearing and is accompanied by a minute granulation, 

 with the development, in many cases, of a microscopic breccia along 

 the planes in question. The lines of shearing as seen under the 

 microscope are not perfectly straight, but while maintaining a generally 

 uniform course often have numerous little anastomosing branch 

 fissures running parallel to them and occasionally crossing from one 

 shear plane to another. The shearing thus takes place along a 

 strip of the rock instead of in a single plane, and this strip is thus 

 filled with a minute calcite breccia. The appearance under the 



