VOLUME XXII NUMBER 3 



THE 



JOURNAL OF GEOLOGY 



APRIL-MAY 1914 



MECHANICS OF FORMATION OF ARCUATE 

 MOUNTAINS 



WILLIAM H. HOBBS 



University of Michigan, Ann Arbor, Michigan 



PART III 



THE FOLDING PROCESS STUDIED IN THE PROFILE — FORMATION OF 



SLIDES 



Imperfect plasticity of folding strata. — Thus far we have dis- 

 cussed the flexures in rocks as though the plasticity of folding sedi- 

 ments were suflicient in all cases to permit without failure the 

 development of both underturned and recumbent anticlines. Such 

 a condition appears to be realized in nature only in the case of well- 

 laminated rock formations, and it is proper here to consider a little 

 more fully the subject of plasticity within compressed sediments 

 under load and at somewhat elevated temperature. 



At the surface of the earth so-called "hard rocks" behave like 

 highly elastic bodies, and the degree to which this property of 

 elasticity is modified by load has prevented a simple mathematical 

 discussion of the subject of folding. Rudzki has stated perhaps as 

 clearly as anyone the reciprocal relations of the properties of 

 elasticity and plasticity in solid bodies. 



The elastic force with which the body resists the deforming force diminishes 

 with time. The time which is necessary for the resisting force to fall to ^ of 

 its original value is called after Maxwell the relaxation time. A body is so 

 much the more plastic the smaller the relaxation time. According to the 



Vol. XXII, No. 3 193 



