606 FRANK D. ADAMS AND J. AUSTEN BANCROFT 



the tube was indicated. Up to this point, the marble, being 

 an elastic body, was undergoing cubic compression, the pressure 

 exerted by the machine and the resistance exerted by the steel 

 collar being equal. The slight distension of the steel tube at a load 

 of 12,000 pounds is due to the elastic deformation of the marble. 

 After each additional increase of 1,000 pounds to the load, exten- 

 someter readings were taken every 30 seconds until four successive 

 readings were identical, that is to say, until no movement that 

 could be registered on the scale took place during a period of 

 2 minutes. The pressure was then increased by another 1,000 

 pounds and a similar series of readings were taken. This was con- 

 tinued until the bulging steel tube showed signs of rupture or was 

 actually ruptured by the movement of the inclosed rock. The time 

 which elapsed between the first application of pressure and the 

 final rupture of the tube, that is to say, the duration of the experi- 

 ment, differed somewhat in the different experiments, but may be 

 said to be about four hours. 



During the time which elapses from the point when the elastic 

 limit of the rock is exceeded to that at which the tube fails, the 

 inclosed rock is undergoing deformation with extreme slowness and 

 by internal movements of one kind or another, which give rise to 

 what may be termed a plastic flow. 



At the commencement of the experiment the column of marble 

 had the form and dimensions represented in the upper half of Fig. 1. 

 When at the conclusion of the experiment the test piece was placed 

 in a lathe and the steel collar was turned off, the specimen of marble 

 was set free. It was still intact, unbroken, and, when tested in 

 compression, was found to be very nearly as strong as a piece of 

 the original marble of the same shape and size. It now had the 

 form represented in the lower half of Fig. 1. 



A photograph of a column of rock, before and after deformation, 

 the rock, however, in this particular case being steatite, is shown 

 in Fig. 3. 



The pressure which was applied to the marble column effected 

 two results. It overcame the pressure (or resistance) exerted upon 

 the sides of the column by the inclosing tube of steel, and it over- 

 came the internal friction developed within the rock during its 



