634 FRANK D. ADAMS AND J. AUSTEN BANCROFT 



was required to bring about a re-establishment of the movement, 

 which, however, eventually resumed its former course. 



BEARING OF THE RESULTS ON CERTAIN PROBLEMS PRESENTED BY 

 THE EARTH'S CRUST 



The experimental results afford a reply to the question pro- 

 pounded by Dr. Gilbert and set forth in the opening paragraph of 

 this paper. They also have a direct bearing on the problems pre- 

 sented by the origin of "decken" and by the theory of isostasy. 



When movement producing deformation is once started in the 

 rock under the influence of tangential thrust, resulting in the break- 

 ing down of its texture, the rock, if deeply buried in the earth's 

 crust, does not on that account offer a decreased resistance to 

 further movement. 



Some experiments by Karman 1 on the deformation of marble 

 under differential pressure have yielded data with reference to the 

 amount of this pressure which must be exerted in the case of marble 

 in order to induce plastic flow in the rock. The data obtained 

 represent maximum results, because in the experiments the pressure 

 was applied rapidly as compared with that which would be devel- 

 oped in any earth movements, and, also, the factor of heat was not 

 taken into account. It must be noted, however, that heat and a 

 very slow application of the deforming force would produce move- 

 ments under lower pressures than those made use of in the experi- 

 mental work. Karman found that, if a column of marble were 

 submitted to a supporting or containing pressure, such as that 

 exerted by the steel tube in our experiments, amounting to 685 

 atmospheres — which would be equivalent to that exerted by the 

 overlying strata at a depth of 2.53 miles below the surface 2 — it 

 would flow uniformly and continuously under a load of 2,870 

 atmospheres applied to the ends of the column. If the containing 

 pressure fell below the value mentioned, that is, if the rock occupied 

 a position in the earth's crust nearer the surface, it would speedily 

 crumble and break to pieces, presenting in this way a failure similar 



1 "Festigkeits Versuche unter allseitigem Druck," Zeit. des Ver. deut. Ingenieure, 

 October 21, ion. 



2 F. D. Adams, "Depth of the Zone of Flow in the Earth's Crust," Journal of 

 Geology, February, 1912. 



