Status of the Theory of Isostasy. 309 



ness to strains of long duration. The importance of 

 this conception in the larger fields of earth dynamics 

 justifies giving this shell of weakness a special name, — 

 the asthenosphere; the stronger zone of compensation 

 above it constituting the earth's crust, or lithosphere. 



In Part VII is taken up "The variation of strength 

 with depth as shown by the nature of departures from 

 isostasy." Conceive the loads on the crust to be of 

 harmonic form, like the form of the broad rollers on the 

 surface of the ocean, as this simplification adapts them 

 to mathematical investigation. Such loads on the sur- 

 face of a solid give a distribution of stress which has 

 been worked out by Gr. H. Darwin. The maximum dis- 

 tortional stress below the mean surface is at a depth 

 equal to 0-163 of the wave-length. The distortional 

 stress is nothing at the surface, the load there giving 

 merely hydrostatic pressure. Below the depth of maxi- 

 mum effect the distortional or shear stress rapidly 

 decreases. 



The irregular loads ascertained to be held by the 

 strength of the crust as measured by departures from 

 isostasy are then transformed into roughly equivalent 

 harmonic loads. It is found that the large stresses are 

 all in the lithosphere, the asthenosphere being compara- 

 tively free from stress. Loads are therefore upheld by 

 the earth in such form as to give stresses on the strong 

 parts of the crust. These results agree with the other 

 evidences regarding lithosphere and asthenosphere. 



In Part VIII are discussed "The physical conditions 

 controlling the nature of lithosphere and asthenosphere. ' ' 



Sources of Hayford's Errors in Interpretation. 



The conclusions as to the perfection of isostasy, as 

 developed in the last topic, stand in contrast to those 

 reached by Hayford. If Hayford's interpretation is 

 wrong, on what false assumptions does it rest? In order 

 to point these out, so far as deflections of the vertical 

 are concerned, the following quotation is given : 



' ' The following table furnishes a measure of the degree of the 

 completeness of the compensation. For a full statement of the 

 meaning of the table and the manner in which conclusions are 

 drawn from it, see pages 164-166 of 'The Figure of the Earth 

 and Isostasy/ etc. 



