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University of California Publications. 



[Geology 



of the table itself which exercises an upward pressure on the 

 book and supports it against the pull of gravity; and there is 

 a general compression of the book itself, for each part must 

 exert a sufficient elastic force on the part above to support its 

 weight. When power is transmitted along a shaft, the force 

 is applied at one end turning the shaft; the elastic shearing 

 force thus brought into play is transmitted along the shaft, each 

 part being slightly twisted on the part beyond it, and thus 

 exerting on it a turning force. Elastic forces are essential to 

 our well-being; we make use of them at every moment of our 

 lives. 



We must look upon the rock under pressure as suffering a 

 certain amount of elastic compression, or distortion. Rock, in 

 common with other substances, is not perfectly elastic, but has 

 some plasticity, and under the long continued action of an ex- 

 ternal force it will gradually change its shape without fracture. 

 We have evidence of this not only in nature but in the labor- 

 atory. When Messrs. Nagaoka and Kusakabe made experimental 

 determinations of the elastic constants of various rocks they 

 found a certain amount of plastic yielding even during the few 

 minutes involved in their experiments. And Professor Adams 

 has succeeded in deforming small specimens of marble without 

 fracture by supporting them in a strong steel tube while sub- 

 jecting them to enormous pressures. If, however, the rock had 

 not been supported on its sides, the great pressures, as in certain 

 methods of testing the strength of materials, would have cracked 

 it into many pieces. Marble slabs over old graves, supported at 

 their ends, have sunk slightly in the middle, not by elastic bend- 

 ing but by a plastic distortion. 



We have then the following facts: the rock in the earth's 

 crust has been subjected to strong forces; it is plastic; it is sup- 

 ported beneath and on the sides by the surrounding rock and 

 above by the weight of the rock resting on it ; therefore, as in Pro- 

 fessor Adams' experiments, it must be slowly deformed; and by 

 this yielding the elastic forces are reduced or at least prevented 

 from increasing as rapidly as they otherwise would. The existence 

 of faults proves that when the forces become too great fractures 

 occur and, moreover, that sudden plastic deformations do not 



