194 WILLIAM H. HOBBS 



kinetic theory one may assume that all bodies are plastic to a certain degree, 

 and that the differences have quantitative value. Obviously for room tempera- 

 ture and atmospheric pressure different bodies possess very different relaxation 

 times. For steel the relaxation time may be some centuries; for wax, parafi&ne, 

 etc., it is so small that the determination of the elastic constants by the usual 

 static methods is difficult to carry out. Plasticity appears in some cases to be 

 dependent upon the magnitude of the deforming force — it is larger and the 

 relaxation time smaller the greater the force. Temperature has the greatest 

 influence upon plasticity, the plasticity increasing with the temperature. 

 Glass, which is at ordinary temperatures so brittle and breakable, shows 

 distinct plasticity above 300° C. (order of magnitude of the relaxation time 

 about a day).^ 



Quite obviously, where not centuries only but very much larger 

 units of time may be involved in the folding process, the load 

 which is necessary to produce plasticity sufficient for folding may 

 be very much less than that indicated by experiments.^ 



There is, however, a way of looking at the subject of potential 

 plasticity as it relates to folding strata, which will give us an insight 

 into the conditions under which failure may occur. The different 

 parts of a fold are subject to internal strains which in the anticlines 

 of relatively late stage differ by large amounts. It therefore occurs 

 that the period covered by the evolution of a fold may be suffi- 

 ciently long to permit the resisting forces within the strata to fall, 

 and local strains to occur without failure except at critical points of 

 maximum deformation. In this event failure will occur at the locus 

 of maximum deformation much as it would in a relatively elastic 

 body, even though the remaining portion of the arch adjusts itself to 

 the stress conditions like a truly plastic body. Our problem is, there- 

 fore, to discover the locus of maximum strain in a growing anticline. 



Contrasted cases of isotropic and anisotropic strata — control of 

 internal strains by lamination. — The problem of representing the sys- 

 tem of internal strains within a developing anticline is complicated 

 by the variations in texture which characterize rock formations. 

 There are, on the one hand, formations such as heavy limestone 



^ Physik der Erde (author's German translation), Leipzig, 1911, pp. 232-33. 



^ See in this connection, F. D. Adams, "An Experimental Contribution to the 

 Question of the Depth of the Zone of Flow in the Earth's Crust," Jour. Geo!., XX 

 (1912), 97-118; L. V. King, "On the Limiting Strength of Rocks under Conditions 

 of Stress Existing in the Earth's Interior," ibid., pp. 119-38. 



