and Rocks under Pressure. 255 



planes. One might expect the cleavage planes of calcite 

 to bear some especial relation to its manner of rupture. 



The specimen was in the form of a single cylinder with 

 a cap, of the same dimensions as the first specimen of 

 quartz. The axis of the cylinder was supposed to be par- 

 allel to the trigonal axis, as were quartz and tourmaline, 

 but subsequent examination showed that the axis of the 

 cylinder was inclined at about 7° to the axis of the crys- 

 tal. The cleavage planes in the original piece were so 

 strongly developed that at first it seemed of little use to 

 attempt the experiment, but the results only confirmed 

 the results with other specimens that original flaws have 

 no effect on the manner of rupture under these special 

 conditions. Apparently the friction produced by the high 

 pressure is sufficient to prevent any slipping on original 

 planes of fracture. 



Three applications Of pressure were made, to 500, 

 1000, and 1500 kg. for 10 minutes each. The most 

 marked effect of pressure was a great increase in the 

 number of internal cleavage planes. The increase was 

 so great as to make the substance partially translucent 

 instead of transparent. The development of cracks put 

 an end to the experiment, the crystal falling apart in 

 handling at atmospheric pressure while preparing for 

 the next test at 2000 kg. The effect of stress on the cavity 

 was not marked, but after 1500 there was unquestionably 

 some flaking-off of the interior, just as for quartz and 

 tourmaline at higher pressures. The material also 

 showed a tendency to slip into the cavity along cleavage 

 planes, but full accomplishment of this tendency was 

 prevented by the geometrical configuration, the parts 

 affording each other mutual support. The inner surface 

 of the wall of the cavity, to a depth of several millimeters, 

 was rendered entirely opaque, evidently because of the 

 grinding together of the material as it was carried toward 

 the center on convergent cleavage planes. The slip pro- 

 duced a definite figure on the plane surface, as shown in 

 figure 6. Within the triangle the development of cleavage 

 planes was much more pronounced than outside of it. 



Slip was accompanied by permanent change of dimen- 

 sions, one of the few examples of it found. The outer 

 diameter decreased from 0-7331 to 0-7310 inches, and the 

 inner diameter by 2-3%. The slipping was in such a direc- 

 tion as to produce an increase of external length from 



