THE STRENGTH OF THE EARTH'S CRUST 427 



the present investigation to the method of multiple working hy- 

 potheses, examination will be made of the paradox which has been 

 drawn between certain conceptions from other lines of investiga- 

 tion and those drawn from this study of crustal strength. 



Having given this introductory presentation on what is con- 

 ceived to be a judicial point of view, we may turn to a review of the 

 conclusions reached in this article. It is pointed out that rigidity 

 is strictly a measure of stiffness; whereas a very different quality, 

 the limit of elastic yielding, or the beginning of flow, is the measure 

 of strength. But mass flowage may take place in a number of quite 

 different ways, according to the nature of the solid and the environ- 

 ing physical and chemical conditions. The elastic limit and hence 

 the strength will differ in the same solid according to the mode of 

 yielding. Four modes may be here enumerated in what is thought 

 to be their order of increasing importance, the fourth mode being 

 that which is conceived as operative especially in the asthenosphere, 

 and serving to maintain the condition of approximate regional 

 isostasy. 



First, flowage may take place rapidly by true plastic or molecular 

 flow, as with lead or white-hot iron, the solid, when stressed well 

 beyond the elastic limit, behaving like a viscous fluid. It is not 

 thought that the terrestrial deformations are often carried on with 

 a rapidity which requires true plastic yielding. In fact, under such 

 rapid stresses as those produced by earthquakes and tides it is not 

 improbable that the strength of the earth may progressively 

 increase with depth. 



As a second and quite different mode, deformation may take 

 place by molar as distinct from molecular shear. In the zone of 

 fracture this is manifested in jointing and faulting and is empha- 

 sized as distinct from rock flowage, but where the fracturing 

 becomes so closely spaced as to result in slicing of individual 

 minerals it passes under the category of granulation. Where 

 carried on at depth there is always some degree of cementation by 

 recrystallization. Deformation by such close-grained fracture 

 without complete loss of cohesion is classed as rock flowage. It 

 is thought to be developed to some degree within the lithosphere, 

 especially by great horizontally compressive forces, but is not 



