420 TERENCE T. QUIRKE 



But in this article there is no design either to emphasize the elas- 

 ticity of the earth as a whole or the liquefaction of local parts, nor 

 on the other hand is there any need to deny the reality of these 

 characteristics. But inasmuch as no term has yet been devised 

 which adequately describes the character of the earth's interior, 

 it is necessary to choose for different purposes different terms 

 emphasizing different phases of the earth's behavior, each admis- 

 sible term being complementary and not contradictory to the 

 others. 



TERRESTRIAL FORCES AND CRUSTAL MEMBERS 



The nature of earth stresses. — The usual argument is that there 

 is a more or less rigid, plastico-frangible, unshrinking crust upon a 

 plastico-rigid, shrinking interior. Between the central sphere and 

 its crust, adjustment is made possible by a zone of almost no 

 strain, above which the earth's crust must undergo strain increas- 

 ing from near zero at the base to a maximum at the surface.^ 

 Within the zone of flow the strain is accommodated by flow, above 

 that by a combination of flow and shearing.^ Any thrust that 

 may be applied in a zone of perfect flow cannot be transmitted as 

 such; it is transmitted hydrostatically. However, it is not prob- 

 able that there is a zone of perfect flow; probably the rock yields 

 under long-continued pressure, whereas, in the manner of tar, it 

 might rupture under a sudden shock. Such shocks are incon- 

 ceivable as affecting this zone of flow, and for purposes of this dis- 

 cussion the zone of flow may be regarded as one in which there is 

 a minimum of vector or directional forces. Movement of the 

 crust over the shrinking interior would tend to produce displace- 

 ment in the zone of flow at an angle. approaching zero, no matter 

 at what angle thrust forces cause rupture in the upper crust.^ 



A perfectly rigid body transmits thrust in such a way that the 

 forces are not dissipated during transmission. This type of body 

 does not obtain in the earth's crust, for rocks are not perfectly 

 rigid materials. Material which is slightly plastic tends to fail 

 near the points of the application of force instead of near the 



' T. C. Chamberlin and R. D. Salisbury, Geology, II (1907), 127-30. 



^ C. K. Leith, Structural Geology (1913), p. 4. 



3 T. C. Chamberlin, "The Problem of Faulting," Econ. GeoL, II (1907), 597-99. 



