206 



SCIENCE 



[N. S. Vol. LIU. No. 1366 



The preiwnderance of environmental evidence 

 seems to indicate that rock flowage is the dis- 

 tinctive kind of movement, but so many 

 qualifications, definitions and assumptions 

 enter into this conclusion that my present in- 

 clination is to keep firmly in mind the com- 

 plex facts of deformation in our zone of ob- 

 servation as a possible key to the interpreta- 

 tion of unseen movements. This attitude will 

 require us to pay more attention than here- 

 tofore to the possibilities of heterogeneous 

 structural behavior at great depths. Particu- 

 larly should we keep in mind the fact that 

 the kind of rock flowage accomplished experi- 

 mentally produces structures which in the 

 earth have sometimes been called fracture or 

 combined fracture and flowage. We may as- 

 sume a downward extension of combined frac- 

 ture and flowage, as observed in the field, and 

 still meet the conditions of flow implied by 

 experiment. 



How Are Stresses Transmitted in the Deep 

 Zone? — In our zone of observation stresses 

 are clearly transmitted by the competent 

 members of the lithosphere. In any area of 

 deformation evidence may usually be found 

 of the control of the structure by one or more 

 competent members. When the notion was 

 widely held that the interior of the earth was 

 molten or fluidal, hydrostatic stress condi- 

 tions were naturally assumed. With the later 

 knowledge that the earth acts essentially as a 

 solid throughout, this view was largely aban- 

 doned in favor of the view that rocks in the 

 deep zone act as rigid competent members 

 capable of transmitting stresses in definite 

 directions. The vector properties of cleavage 

 and other structures supposed to develop in 

 this zone were cited to indicate the definite 

 orientation of stresses. It does not follow 

 from this, however, that pressure conditions 

 were or are not hydrostatic, especially under 

 slow movements. Rocks under compression 

 from all sides greater than their crushing 

 strength seem to transmit stresses in a man- 

 ner suggesting approach to hydrostatic con- 

 ditions of pressure. When the stress differ- 



ences are such as to require it, there is 

 movement in the direction of easiest relief. 

 The stress as reflected by the movement 

 would seem to have been transmitted in a 

 defiinite direction, and yet the pressures may 

 have remained hydrostatic. If we were to 

 imagine a volume of liquid deep below the 

 surface subjected to differential stress suffi- 

 cient to deform its containing walls, it is clear 

 that the movement would be in the direction 

 of easiest relief, notwithstanding the hydro- 

 static conditions within the liquid. Periodic- 

 ity of movement is possible imder this con- 

 ception. Rock structures indicate movement 

 only, not necessarily the inherent stresses. 

 Movement of rocks imder the conditions sup- 

 posed to obtain deep below the surface seems 

 likely to be at least in part a matter of relief 

 of materials so contained between rigid mem- 

 bers that the direction of escape is definitely 

 oriejited. Of course this supposition assumes 

 that on some scale, small or large, there are 

 units of mass competent to act as retaining 

 walls for materials acting under hydrostatic 

 pressure. If all the mass in the deep zone 

 were under hydrostatic pressure, the retaining 

 walls might be regarded as the solid sheU 

 above, inequalities in the competence of which 

 would control the movements in the direction 

 of easiest relief. However, rock structures, 

 such as cleavage and folds, with vector ar- 

 rangement of the sort observed near the sur- 

 face and of the sort supposed to exist below, 

 tell us only of the direction of movement and 

 fail to indicate whether the stresses are hydro- 

 static or otherwise. 



CONCLUSION 



Within the zone accessible to observation 

 movements of rock masses are accomplished 

 by fracture and flowage. These processes may 

 be distinct and separate, or so interrelated as 

 to make definition diflicult. The zones of 

 movement are many, their positions and atti- 

 tudes diverse. In general they indicate shear- 

 ing or grinding movements between rock 

 masses, accomplished both by fracture and 

 flowage, and caused by stresses inclined to 



