12 R. T. CHAMBERLIN AND W. Z. MILLER 
eliminated. One side of the box was constructed so that it could be 
removed as often as desired during the course of each experiment 
in order to note the nature and progress of the deformation and to 
photograph the structure developed. Pressures were applied by 
means of a 14-inch steel screw turned by hand, operating upon a 
lever arm of 24 inches. At the opposite end of the box the other 
pressure block was moved by a 1-inch screw. This was much less 
frequently used. These screws were capable of developing such 
thrusts that one of the chief difficulties was to secure an apparatus 
of sufficient strength to withstand the stresses to which it was 
Fic. 4.—Pressure box with a movable thrusting block at each end. The detach- 
able side has been removed to permit a view of the interior. 
subjected. Various strengthening devices were employed. If 
another apparatus were attempted, it would be constructed entirely 
of steel. 
Materials —Before attempting experiments upon a succession 
of strata of varying competency, as would be the case in nature, 
it was essential to determine the effect of compressive stresses on 
homogeneous material. Both rigid and plastic materials were com- 
pressed in the crushing machine to determine their different 
behavior under stress. For rigid material either plaster of Paris 
or a mixture of plaster with a small amount of clay was used. As 
a result of many trials it was found that to secure rigid, brittle 
material, such as best illustrates faulting, a mixture of three parts 
of plaster and one part of clay gave the best results. To develop 
greater plasticity the proportion of clay was steadily increased, and 
in various cases a certain proportion of sand was added until there 
was as much as two parts of clay and sand to one part of plaster. 
