52 G. F. BECKER FINITE STRAIN IN ROCKS. 



years. This does not imply that there is no true elastic limit, but only 

 that it is lower than brief laboratory experiments would lead one to 

 suppose. Were there no elastic limit, it seems to me that we should 

 find, for example, quartz crystals in vugs among the more ancient rocks 

 sensibly distorted by their own weight. 



Usually then the line of a simple, direct pressure which has produced 

 two or four systems of fractures in large rock masses, or in the pebbles 

 of conglomerates, will he found to bisect the obtuse angles between the 

 fissures as the mass now stands. In any case, where it is suspected that 

 the line of force bisects the acute angles between fissures, the slickensides 

 should be minutely examined to ascertain whether they show reversal of 

 motion, and all the attendant phenomena should be investigated. 



When a simple pressure on a rock mass increases very gradually, it 

 will for some period exceed the elastic limit of the rock and fall short of 

 the ultimate strength. Flow must then take place. The only feature of 

 this flow which will reveal itself to observation will lie the relative move- 

 ments of adjoining particles. Hence, although the path in space of each 

 particle will lie hyperbolic,* the evidence of movement will indicate rela- 

 tive transfer of adjoining particles in opposite directions along lines of 

 maximum tangential strain. The energy of this relative movement will 

 evidently increase with the excess of the pressure above the limit at which 

 flow begins, sometimes called the limit of solidity. 



Thus, if one supposes the pressure suddenly to surpass the limit of 

 solidity and then to be kept constant, the mechanical effects of the rela- 

 tive motion (and the chemical effects attending the expenditure of energy) 

 will be very pronounced on the lines on which flow begins. As the pro- 

 cess continues and the stress diminishes with the increase of the area of 

 the mass, the lines first affected will make an increasing angle with the 

 line of force, while the new fibers of the material which are forced into 

 the direction of maximum strain will be less and less affected. 



The result will at least resemble schistose structure and will be marked 

 by the presence of lines of relative movement intersecting one another at 

 very acute angles. In the case of direct uniform pressure there will be 

 four such sets, each set at a large angle to all the others. 



If the load were to increase in the same proportion as the area of the 

 loaded mass, so that the stress would be kept uniform, an indefinite 

 amount of flow might be produced, provided that the rock is not hardened 



♦During flow there, is no progressive change of volume. Hence, a point for which at the incep- 

 tion of flow x = 1, y = 1, will be moved to a point x', i/, and .<■"- y' — 1. The curves of this form are 

 sometimes called the lines of flow They would be more aptly called lines of absolute movement. 

 They should carefully hi' discriminated from the lines of relative movement, which are straight. 

 The hitler are the only ones of which the deformed mass can give direct evidence. 1 n i he case of 

 simple shear the lines of absolute movement are simple hyperbolas asymptotic to the axes. 



