Nature and Bearings of lsostasy. 



98' 



The experiments of Adams have shown conclusively 

 that for _ the conditions of temperature and pressure 

 which exist in the outer crust the strength greatly in- 

 creases with depth up to at least 15 or 20 kilometers. 

 The degree of maintenance of isostasy through geo- 

 logical time in spite of opposing agencies shows, on the 

 other hand, that a deeper shell in the earth is very much 

 weaker than the surface rocks. Under the supposition 

 that various geological agencies have loaded the crust 

 to near the limit of its strength and that the condition 

 of isostasy is due to failure beyond that limit, the 

 strength may be determined for various depths and an 

 approximate curve of strength may be drawn. The shell 

 of weakness revealed by isostasy as existing below the 

 shell of isostatic compensation is thought to have very 

 fundamental relations to the architecture of the earth, 

 the mode of expression of its internal forces, and to the 

 origin of magmas. As weakness under permanent shear- 

 ing stress is its conspicuous feature, it is named the 

 asthenosphere, the far stronger crust above being the 

 lithosphere. Without such an asthenosphere it is thought 

 there could be no notable degree of isostasy. 



How do these conclusions regarding the physical 

 nature of the asthenosphere correspond with the evi- 

 dences of high rigidity derived from the small degree 

 of yielding to tides and earthquakes ? At first sight there 

 seems to be a sharp disagreement between two well- 

 established lines of evidence. This paradox appears to 

 be due, however, to a confusion of the conceptions of 

 rigidity and strength. Rigidity is increased with pres- 

 sure and increase in rigidity corresponds to a rise in 

 the moduli of elasticity. Strength, on the other hand, is 

 measured by the elastic limit, beyond which solid flowage 

 takes place. Under permanent load and temperatures 

 near fusion the solid yields to moderate permanent 

 stresses by recrystallization without the moduli of 

 elasticity being changed. Under these conditions high 

 rigidity is compatible with low elastic limit. 



In conclusion there will be discussed briefly, in answer 

 to the questions raised in the introduction, the bearings 

 of isostasy on the hypotheses dealing with the origin 

 of ocean basins and continental platforms. Two of these 

 hypotheses are at present more particularly before the 

 public ; they have been formulated by two of the leaders 



