THE STREXGTH OF THE EARTH'S CRUST 429 



verge by rising, analogous to the draining of melted water from 

 glaciers; uniting as rivulets unite into rivers, and rivers discharge 

 into lakes. Xo continuous lava stratum or large reservoirs of lava 

 couldj under the terms of this h^-pothesis, be expected to exist 

 ^thin the asthenosphere. Its verv- weakness would prevent it 

 from acting as a containing vessel for holding large volumes of any 

 5^iid wliich, for any cause such as a lower specinc gra\-it\- of the 

 ~-ii i 7. ji; ;e, would tend to rise. The evidence of earthquake \abra- 

 tions and of resistance to tidal deformation further supports the 

 vir" ?_;' hr itthr' sphere is not a liquid or even a truly \tscous 

 z.i' r 1'- —'-t ..-i^r rand, only in the Hthosphere would be found 

 the strength needed for the storage of magma in volumes until the 

 limit of its strength as a containing vessel was reached. 



Partly guided by observation upon the metamorphic rocks, 

 partly by theories of the nature of deformation at great depths, 

 the argument leads to conclusions on the mode of \-ielding within 

 the different levels of the crust. First, the outermost zone is 

 observed to be a zone of fracture, weak in comparison with the 

 thick zone below. This, the second zone, is the zone of strength and 

 \-ield5 by flowage, but flowage which is characterized by granulation 

 as the dominant, by recrv-staUization as the subordinate, mode. 

 The expenditure of energy- for a given deformation is here a maxi- 

 mum. In the third zone, the asthenosphere, on the contrarv*, 

 flowage is conceived as taking place with but little expenditure 

 of energy', by a ready recrjstaUization at the temperature of 

 primary crystallization of magmas. Those contorted granite- 

 gneisses seen espedsJiy in the Archean rocks, which are regarded 

 as deformed during the final stages of oystallization, exhibit 

 locally in the outer crust the conditions which it appears may 

 permanently prevail within the asthenosphere. 



SEcnox A 



RELATIONS 3ZI~/.^rx RIGIDITY. STREXGTH AST) IGNEOUS 



ACTI\TTY 



DISTINCTIOX5 IS PHYSICAL PROPERTIES RELATED TO 5TREXGTH 



Elasticit}' is of two natures: that of volume and that of form. 

 The first is possessed by matter in either the gaseous, liquid, or 

 solid state; the second is possessed by solids only and is associated 



