and Rocks under Pressure. 257 



being a well-marked rhombus in section, with evidently 

 some connection with the crystalline system. The long- 

 axis of the rhombus was parallel to the direction of the 

 white streaks in the original crystal. This substance is 

 unusual in that no fissures whatever accompanied the 

 erosion. Apparently, therefore, the mechanism of rup- 

 ture by flaking-off is entirely independent of the mechan- 

 ism of formation of cracks. As usual there was no 

 change of external dimensions. 



After exposure to 5000 it was exposed again to 3000, to 

 see whether the flaking-off would continue at a lower 

 pressure after having started at a higher pressure, and 

 also to find whether loss of circular cross section in the 

 cavity would result in easier rupture. There was prac- 

 tically no effect from the second application of 3000; a 

 slight quantity of dust had flaked off the inner surface, 

 but no more than on the first trial. 



The change of internal diameter as measured by the 

 solder plugs was 0-2% per 1000 kg., which is of the same 

 order as for quartz. Apparently the elastic constants of 

 feldspar have not been measured, so there is no chance 

 to compare this value with calculated values. 



Barite. — This was a translucent specimen with numer- 

 ous flaws, which however did not affect the behavior 

 under pressure. The specimen was of the form and 

 dimensions of the second specimen of quartz. This was 

 exposed to 1000 kg. for 4 hours, to 2000 for 17 hours, to 

 3300 for 6 hours, and to 11,400 for 39 hours. 



The first two applications of pressure had no percepti- 

 ble effect. At 3300, however, the cavity was partly dis- 

 integrated in the usual fashion, but not tightly packed 

 with sand. The eroded region was a rhombus in section, 

 rather more nearly square than for feldspar, and around 

 this was another rhomboidal region, equal to the diameter 

 in width, within which the original translucent material 

 had changed to an opaque white like marble, evidently due 

 to internal slip. There was no change of external 

 dimensions. 



It was evident that a pressure only slightly above 3300 

 would produce the usual complete disintegration, and 

 occasion was therefore taken to answer a question sug- 

 gested by much of the previous work, namely, what hap- 

 pens to the sand in the cavity when pressure is raised 

 very far above the disintegrating pressure? It is hardly 



