688 T. C. CHAM BERLIN 



In considering stress effects, it is necessary scrupulously to 

 distinguish between hydrostatic stresses which operate equally on 

 all sides of a given unit and so only produce compressive and like 

 effects, and differential stresses which promote movement and 

 change of form. The effect of differential stresses on the solid 

 parts of the earth is primarily to produce strains; the effect on 

 liquid parts is primarily to produce flow and relocalization. And 

 so by reason of this difference of effect, a general differential 

 stress on any large part of the earth is apt to become locally sub- 

 differentiated when solid and liquid parts are intermixed, especially 

 if the liquid and solid states of these parts are partially inter- 

 changeable because their temperatures lie so close to the line of 

 equilibrium between solidity and liquidity. Tensional strains 

 promote liquefaction in bodies constituted as most rocks are; 

 compressive strains resist liquefaction in such bodies. And so 

 general differential strains co-operate with temperature in pro- 

 moting or in restraining the passage of matter from the one state 

 to the other according to the nature of the strain and thus have 

 some influence in directing and facilitating movement as well as 

 in forcing it. 



Some of the differential stresses in the earth are essentially 

 fixed and constant, such as the direct pressures that arise from the 

 action of gravity. These stresses range from one atmosphere 

 at the surface to about three million atmospheres at the center. 

 Such pressure s tend to force lighter bodies toward the surface while 

 heavier bodies seek the center in ways so familiar that we need 

 not dwell on them, nor on the fact that, since molten rock is usually 

 lighter than the same rock in a solid state, this static differential 

 stress of gravity presents a general condition that favors the 

 ascent of liquid rock. So also the incorporation or generation of 

 gases in liquid rocks tends to lessen the specific gravity and increase 

 the mobility and hence the gaseous element adds another general 

 influence that favors ascent. 



In addition to these very general and persistent stresses, more 

 special differential stresses have arisen at various times from 

 inequalities of accession, from transfers of matter, from loss of 

 heat, and from other varying agencies, and these have been present, 



