DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 509 



dence as to the character of the movement which had taken place 

 could be obtained by a microscopic study of the crushed mineral. 

 The sections in both cases were cut parallel to the orthopinacoid, 

 while, for purposes of comparison, a third section running in the 

 same direction was prepared from one of the original uncrushed 

 crystals. Under the microscope the crystals which had been sub- 

 jected to compression were seen to be traversed by a number of minute 

 cracks, and also showed in the much-crushed portions faint strain 

 shadows, when examined between crossed Nicols. It is evident, 

 therefore, that under the conditions of the experiment, the orthoclase, 

 while probably displaying a very slight plastic movement of the nature 

 of twisting, as shown by the slightly uneven extinction produced by 

 the pressure, moves almost entirely by fracture and granulation. 

 This agrees with the deportment of orthoclase as observed in highly 

 deformed rocks in the earth's crust, the mineral in these rocks being 

 usually granulated or recrystallized under conditions of differential 

 pressure. 



Magnetite. — A perfectly symmetrical octahedron of this species from 

 Mineville, New York, was, as mentioned above, embedded in alum 

 and submitted to pressure, with the diopside crystals whose behavior 

 has already been described. On dissolving the alum, the magnetite 

 was found to have been broken to pieces, the fragments having the 

 form of little plates which had separated from the crystal parallel to 

 the octahedral faces. 



Pyrite, — The pyrite employed was from the Saratoga Mine, 

 Gilpin County, Colorado. The fragment selected had the form of 

 a half cube, showing the crystal faces, with a surface of fracture on 

 one side. The specimen, with the edge of the cube upward, was 

 embedded in alum in a copper tube with a brass plate at either end. 

 Pressure was then applied, the deformation of the tube occupying 

 17 minutes, and the maximum load attained being 43,000 pounds 

 (19,404 kilos). No sounds whatever issued from the tube as the 

 deformation went forward. On removing the highly bulged tube 

 from the press, it was found that the edge of the pyrite crystal, referred 

 to above, had passed completely through the brass plate and had cut 

 into the iron head plate of the press, the edge, however, remaining 

 practically intact. On dissolving away the alum, it was found that 



