and Rocks under Pressure. 259 



with molecules one layer deep. As far as any present 

 evidence goes, it is not unreasonable to suppose that the 

 sand formed by stress in a cavity originally entirely free 

 from gas would ultimately be fused together again to a 

 coherent mass. 



This completes the experiments on single crystals. As 

 being of geological interest, several experiments were 

 made on rocks and other substances. 



Porphyry. — The two cylinders of this material were of 

 approximately the same shape and dimensions as the 

 second quartz. They were exposed to 1000, 2000, 3000, 

 4000, 6000, and 7000 kg. for 10 minutes each. Up to 4000 

 there was no perceptible effect; at 6000 a slight flaking 

 off was perceptible at the mouth of the hole, and at 7000 

 failure was complete by flaking-off. The eroded regions 

 were roughly triangular prisms, the prism in one piece 

 being a prolongation of that in the other. There was no 

 viscous flow of the outside of as much as 0-0001 cm. 

 and no cracks whatever in the solid mass, even in the 

 neighborhood of the eroded region. The mean density of 

 the sand was only 0-31. Doubtless if the pressure had 

 been continued longer, the sand would have become more 

 tightly packed. 



The elastic decrease of diameter of the inner hole was 

 at the rate of 0-14% for 1000 kg. This is considerably less 

 than that of quartz, showing that probably the mean elas- 

 tic constants are considerably higher. Failure by erosion 

 occurred at a lower stress than for quartz, however. Of 

 course this is just as one would expect; a rock would 

 have elements of weakness not possessed by an individual 

 crystal. 



The stress at the inside at the rupture point may be 

 computed for an isotropic substance without knowing 

 the elastic constants,, and was approximately 14,000 

 kg/cm 2 . Engineering tests on porphyry give a com- 

 pressive strength under ordinary one-sided crushing 

 tests varying from 1000 to 2600 kg/cm 2 . The excess 

 above the value computed from ordinary tests is there- 

 fore high, although not so extreme as for quartz. 



Andesite. — This was a very fine-grained and perfect 

 specimen, of the form and dimensions of the second speci- 

 men of quartz. Dr. Becker remarked that the results 

 would be of particular significance because the composi- 

 tion of this rock is the mean composition of the crust of 

 the earth. 



Am. Jouk. Sci— Fourth Semes, Vol. XLV, No. 268.— April. 1918. 

 19 



