INTERNAL FRICTION IN ROCKS 635 



to that which is obtained in testing building stones in the laboratory. 

 On the other hand, if the containing or supporting pressure is 

 increased, the load required to produce deformation rapidly 

 increases also, and the experiments seem to indicate that with a 

 containing pressure of about 10,000 atmospheres, which would be 

 equivalent to a depth of about 22 miles below the surface, it would 

 be impossible to make the marble flow, except under a pressure 

 which would be simply colossal. 



Since with the increase of resistance to tangential thrust, that 

 is, with increasing depth below the surface of the earth, the amount 

 of such thrust required to produce movements in the earth's crust 

 increases rapidly, it is evident that the great movements of adjust- 

 ment by rock flow or transference of material in the earth's crust 

 from one point to another — -other than the transference of rock in 

 a molten condition — must take place comparatively near the sur- 

 face. That is, beneath the zone of fracture where adjustment takes 

 place by faults and overthrusts — in the zone of flow — movements so 

 far as they are determined by pressure are effected with an ease 

 which increases rapidly in proportion to their nearness to the 

 surface. 



It would seem, therefore, that it is in the upper part of the zone 

 of flow only that the great "decken," as, for instance, those which 

 are developed in the Alps, are produced. This explains the fact 

 that in the mountain range in question it is the upper "decken" 

 which have moved more rapidly and have extended farther than 

 the lower "decken," where the rock is under the increased load and 

 is consequently much less plastic. 



Since with the increase of depth there is a rapid increase in 

 rigidity of the rocks of the earth's crust, it is not difficult to under- 

 stand how it is that, while great movements may take place near 

 the surface of the earth in the upper part of the zone of flow, the 

 globe itself is "more rigid than steel or glass." 



The experimental work also affords at least a first approxima- 

 tion to the determination of the dimensions of the forces which are 

 required in order to effect deformation in the earth's crust in the 

 case at least of the chief types of rocks which make up the crust 

 in question. 



