INCOMPETENCE OF ISOSTATIC STRESS TO PRODUCE MOTION 378 



at a depth of 5.8 miles below the surface have so gained in strength, be- 

 cause of confining pressure, that they are competent to support a column 

 of rock 23.3 miles high; that is, they are four times as strong as the load 

 which rests on them, and, even though the weight of a mountain range 

 two or three miles high were added or subtracted, there would be no 

 movement in the rigid foundation, except an elastic one. While it is ad- 

 mitted that a sufficient rise of temperature lowers the internal friction 

 of rocks until they may even become liquid, it can not be assumed that 

 that condition is originated except at depths exceeding 30 miles, and 

 even there only as an occasional and not as a permanent condition. Thus 

 the outer portion of the lithosphere must be regarded as a very rigid and 

 adequate foundation for all the minor irregularities of the earth's surface 

 of the dimensions of mountain ranges. 



Hayford answers this argument by an appeal to the slow yielding of 

 substances under even very moderate stress, provided the stress be con- 

 tinued long enough. He appeals to geologic time, and certainly no geol- 

 ogist is likely to dispute his claim that there is enough of it. Neverthe- 

 less, the facts of geology do not accord with his argument, for it can lie 

 shown that the earth's surface has been stable during very long periods, 

 whereas instabiKty is characteristic of relatively short epochs. This is 

 the reverse of what should be the character of the movements if slow 

 yielding to fatigue were the manner of deformation. 



To illustrate: Becker suspended long steel tapes in the Washington 

 Monument and subjected them to strain by attaching weights which were 

 far within the elastic limit. There was an immediate elongation, which 

 represented the elastic stretch of the tapes. Then, during months, the 

 tapes continued to elongate and took a set — that is to say, they did not 

 recover when the weights were removed. The nature of this deformation 

 was a slow, continuous yielding, involving a molecular readjustment that 

 might be designated by the word flow. It was in no sense paroxysmal, 

 nor did the elongation due to fatigue take place only during a brief 

 episode after a prolonged period of immobility. 



Earth movements, in contrast to those of the steel tapes, are markedly 

 concentrated in brief epochs. For instance, we speak of the elevation of 

 the Appalachian Mountains at the close of the Paleozoic as the "Appa- 

 lachian Eevolution." The phrase indicates the very great development 

 of activity in mountain growth which followed the long period of stability 

 that had endured during the Carboniferous. Similarly, the Jurassic, 

 Cretaceous, and Eocene were, for our Atlantic coast, periods of extra- 

 ordinary stability, during which the land was reduced to a very low pene- 

 plain ; yet in the Miocene and subsequent epochs down to the present 



