DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 519 



place in planes of fracture, for the rock remains intact. The 

 movement is that of a very stiff but nevertheless plastic mass. 



In all the experiments just described the marble was completely 

 surrounded by the embedding material. As the experiment proceeds, 

 however, this flows away from about the top and bottom of the 

 column or sphere where the pressure is greatest, and the rock is thus 

 really caught between the upper and lower press plates of the machine, 

 great lateral pressure however being at the same time exerted by the 

 alum and its inclosing copper tube, and the conditions of differential 

 pressure being thus secured. 



Other experiments, however, show that a certain, though less 

 pronounced, deformation will be produced if the marble remains in 

 the middle of a mass of alum and is submitted only to pressure exerted 

 by the moving alum itself, so that, if the differential pressure be of 

 a high order, the harder limestone will be deformed by the movement 

 of the surrounding but relatively softer matrix, provided there is not 

 too great a difference in the relative stiffness of the two. 



As a matter of interest an experiment was made to ascertain 

 whether it would be possible to drive a nail through a mass of marble, 

 under conditions of differential pressure such as those described above. 

 A short nail, o . i inch in diameter and with a broad, flat head, was 

 made of hardened steel. Two disks of steel were then prepared, 

 through each of which a hole the size of the nail was drilled. A disk 

 of marble 0.2 inch in diameter was then placed between the two 

 steel disks and the nail was placed in the hole in the upper steel disk, 

 so that it rested in a vertical position on the center of the marble plate. 

 The whole was then placed in a copper tube and embedded in alum 

 in the usual manner. Pressure was then applied and the whole was 

 squeezed down. On dissolving away the alum the upper steel plate 

 was found to have been distinctly bent by the moving alum, the head 

 of the nail was broken into small pieces, but the shank of the nail 

 had passed completely through the marble disk, making a clean, 

 well-defined hole in the upper portion and shoving out a little conical- 

 shaped piece of marble before it on the lower surface of the disk. The 

 marble showed no trace of crack or fissure — the steel had passed 

 directly through its substance. A photograph of the plate upon the 

 completion of the experiment is shown in Plate VI, Fig. a, a new 



