DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 511 



alum, the greater part of the crystal was found to have been reduced 

 to a fine powder. There were a few larger fragments surviving but 

 these showed no signs of distortion. The fragments when examined 

 between crossed Nicols were found to be perfectly isotropic. The 

 garnet, in fact, had been crushed and displayed no traces of plastic 

 deformation, 



DEFORMATION OF ROCKS 



Seeing, as has been shown, Kick's method is not adapted for the 

 development of plastic deformation in materials which are very hard, 

 the rocks selected for examination were chiefly of the softer kinds, 

 marbles, limestones, and dolomites of different varieties. The harder 

 rocks were, however, represented by a typical granite. In these experi- 

 ments columns of the rock were, usually employed. These were 

 1 . 575 inches (4 cm.) long and usually o. 787 inch (2 cm.) in diameter, 

 with a smooth and generally a polished surface. The copper tubes 

 employed were somewhat larger than the column so that the alum 

 might completely inclose the latter, and had an internal diameter of 

 1 .06 inches (27 mm.) and a wall thickness of o. 125 inch (3 . 175 mm.). 

 In some cases, as will be mentioned, cubes, prisms, and spheres of 

 the rock were also deformed. 



A. marble: CARRARA, ITALY 



This is the same white statuary marble which was employed in a 

 former investigation and described in a former paper. ^ In the case 

 of the large spheres, however, a somewhat commoner variety from the 

 same locality was employed, since blocks of the statuary marble of 

 requisite size for the preparation of these spheres could not be 

 obtained. 



I. COLUMNS 



Columns of the marble were first used and the effect of various 

 kinds of embedding material was studied. 



{a) Deformation with paraffine as an embedding material.- — -The 

 maximum load required for deformation of the tube with its inclosed 



I F. D. Adams and E. G. Coker, "An Investigation into the Elastic Constants of 

 Rocks, More Especially with Reference to Cubic CompressibiUty," Carnegie Institution 

 0} Washington, Publication No. 46 (1906), 26. 



