1(> G. F. BECKER— FINITE STRAIN TX ROCKS. 



attention to this property, which possibly plays some pari in the interior 

 of the earth if not near the surface. 



Plastic flow certainly plays an important part in geological mechanics- 

 The motion of glaciers is known to be in part ascribable to it. and it is 

 clearly evinced in the details of rock structure. At greal depths below 

 the surface a partial gradual relict' of strain in any rock mass will bring 

 to bearagrad.ua! increase of stress difference, which may be considered 

 entirely indefinite in amount. Granting, then, that there is no infinitely 

 brittle rock or no rock in which the ultimate strength tails short of the 

 limit of solidity, flow must ensue at great depths whenever a sufficient 

 relief of strain occurs. No geologist need- to be reminded of the instances 

 pointing to such flow. They are innumerable and most various. 



If a mass capable of plastic flow is suddenly subjected to ;i definite 

 load greater than it can hear without flowing, one-third of the load will 

 immediately he employed in compression and the process of flow will 

 produce no further modification of the volume. Flow is thus continuous 

 shear. 



The shearing process must take place along certain lines, and these 

 must he the lines which are first strained beyond the limit of solidity. 

 In other word-, flow must take place along the lines of maximum tan- 

 gential strain discussed in a former part of this paper. ami which by I 10) 

 stand at an angle 90° — <» to the line of a simple, direct pressure. When 

 the load is of fixed amount, the stress will gradually diminish as the mass 

 flattens out; so that the last lines of How will make a smaller angle with 

 the line of force than the earlier ones. A greater amount of flow would 

 occur along the earliest lines affected. If the mass were of such a char- 

 acter as to show evidences of the relative motion after equilibrium had 

 been reached, a cross-section of it would reveal a structure at Least com- 

 parable with schistosity. the flatter lines being more pronounced than 

 the steeper ones. 



Relation of plastic Solids to Fluids. — Let S he the resistance which a 

 plastic solid opposes to distorting stress at the elastic limit, and let u he 

 the stress which would he required to produce the unit shear it' the mass 

 were perfectly elastic (or, in other words, the modulus of rigidity); then 

 if stress is proportional to strain. S/n is the shearing strain which the 

 mass experiences at the elastic limit, and any greater -train would be 

 accompanied by flow. If the mass continues to How as long as the stress 

 is maintained above the fixed limit S/n, the substance is known as per- 

 fectly plastic. 



If S is infinitesimal, the mass will yield to any shearing stress, however 

 small. Such a mass, resting on a level surface, would spread out to a 

 layer of infinitesimal thickness, much like a fluid. It does not follow. 



