MECHANICAL STATE OF THE ISOSTATIC SHELL 253 



But crushing is not possible vvitliin the earth's mass in tlie way in which we 

 see it at the surface. To crush is to separate into incoherent particles, and 

 irresistible confinement, itself due to the pressures which are greater than 

 coherence, holds any deep-seated rock-mass to its coherent volume. In this 

 condition, confined under pressures greater than its crushing strength, a sub- 

 stance may be said to be latently plastic. The cohesion between its particles 

 is unimpaired ; fracture or crushing into separate grains is impossible for 

 want of space ; but change of form may be induced by a sufficient disturbing 

 force, and such change is plastic flow." 



Van Hise and Hoskins subsequently estimated the depth at which, 

 under the most favorable conditions for strength, a small cavity might 

 remain open as 10,000 to possibly 12,000 meters — that is, below that 

 depth the stress-difference due to the superincumbent load would be 

 greater than could be permanently resisted and the rock would flow. 

 This estimate was based on the assumption that the strength and elastic 

 properties of rocks under great pressure are comparable with those of 

 rocks under surface conditions. 



Adams and Bancroft,^ by their brilliant experimental researches, have 

 demonstrated the accuracy of Gilbert's postulate and have shown that 

 rocks do increase in strength under pressure. The gain in strength is, 

 moreover, at a rate far greater than would have been supposed, and the 

 zone of flow must lie correspondingly deeper than was estimated, except 

 as heat may modify the viscosity of rocks. 



Unfortunately, the experiments are insufficiently numerous to yield 

 quantitative data by which the strength of rock at any desired depth might 

 be determined. They do, however, suggest limiting values, which, with a 

 full appreciation of their approximate character, are indicated in plate — . 



The nature of Adams' experiments is shown in the flgure taken from 

 his paper (figure 1, page 602). Pressure applied to the rock column 

 forced it to expand against the resistance of the steel jacket in which it 

 was inclosed. According to Adams : 



"The pressure which was applied to the (rock) 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 overcame the internal friction developed 

 within the rock during its change of shape." ® 



The resistance of the steel tube was determined by an independent ex- 

 periment, in which tallow was substituted for the rock, and it was thus 



' L. M. Hoskins : Flow and fracture of rocks as related to structure. U. S. Geol. Sur- 

 vey, 16th Ann. Report, 1896, pp. 8.58-859. 



C. R. Van Hise : Principles of pre-Cambrian geology. Ibid., pp. 591-593. 



8 F. D. Adams and .7. A. Bancroft : On the amount of internal friction developed in 

 rocks. Jour, of Geol., vol. xxv, 1917, pp. 597-637. 



•Ibid., p. 606. 



