ISOSTASY OF THE RIGIDITY OF ROCK 325 



experiments of Adams and Bancroft. 15 They showed that, under the 

 conditions of their experiments, the differential pressure "required to 

 produce a pronounced movement" in the rock increased markedly with 

 the hydrostatic pressure. Under hydrostatic pressure equivalent to the 

 weight of -1.2 miles of rock, the deformation of marble required an addi- 

 tional differential pressure equal to the weight of 11 miles of rock, and 

 granite required more than twice as much. These forces are so great 

 that, if the results of the experiments were directly applicable to the 

 earth, the principle of isostasy would have to be abandoned. But are they 

 directly applicable? 



High temperature is very efficient in reducing the strength of solids; 

 the experiments of Adams were carried out at ordinary temperatures. 16 

 Time is also a very important factor; the longest of the experiments 

 lasted only six hours. Every one who makes delicate measurements de- 

 pendent on elastic forces knows the difficulty encountered on account of 

 permanent sets, and that the amount of these sets depends on the duration 

 of the stress. Adams found that a piece of marble in the shape of a lathe, 

 supj)orted at its ends like a beam and weighted in the middle, yielded con- 

 tinuously after a certain weight had been added. 17 (The tensions brought 

 into play may have had some influence on the result.) Indeed, the slow 

 yielding of many substances to continued forces, much smaller than are 

 needed for deformation in a short time, is too well known to be dwelt on ; 

 and it must be remembered that the total displacement is the sum of the 

 distortions of the parts and may be considerable even when the latter are 

 small. 



Let us turn to geological observations. Strata have been intensely 

 folded and distorted ; marble slabs in cemeteries, supported at their ends, 

 have bent down in the middle in the course of one or two centuries, and 

 crystalline rocks have been greatly compressed. It matters not that the 

 distortion of crystalline rock has been referred by petrographers to re- 

 crystallization; the only important thing in this connection is that con- 

 siderable displacements have taken place. Some of these movements may 

 have required great forces, but the marble certainly did not. Molten lava, 

 which surely comes from a depth of several miles, and reaches the earth's 



15 Frank D. Adams and J. Austin Bancroft : On the amount of internal friction devel- 

 oped in rocks during deformation and on the relative plasticity of different kinds of 

 rocks. Jour. Geol., 1917, vol. xxv, pp. 592-637. 



16 In his earlier experiments, Adams used temperatures as high as 550 degrees. He was 

 then studying the effect of pressure in closing small cavities in rock. An experimental 

 contribution to the question of the depth of the zone of flow in the earth's crust. Jour. 

 Geol., 1912, vol. xx, pp. 97-118. 



17 Frank D. Adams and Ernest G. Coker : The elastic constants of rocks. Carnegie 

 Institution of Washington, Publication 46, 1906. 



