272 B. WILLIS DISCOIDAL STRUCTURE OP THE LITHOSPHERE 



geneity," I am unable to follow him in the statement that it appears 

 "quite possible that underflow in a mobile layer might affect a practically 

 perfect adjustment for differences of density above the layer, so as to 

 bring crustal densities and crustal relief into harmony, and yet leave 

 uncompensated the differences in density of the nucleus." The part of 

 the statement which I have italicized appears to overlook the fact that 

 the mobile layer would adapt itself to any underlying inequalities of 

 gravitative attraction, not as readily, but in the same manner as the 

 ocean does, and that at the upper surface of the mobile layer the intensity 

 of gravitation would become, by adjustment, everywhere the same. 



This conclusion agrees with the concept of Hayford and Bowie as to 

 the uniformity of gravitative stresses at the base of the zone of compen- 

 sation, but it recognizes diversity of constitution, and therefore inequal- 

 ity of local stresses, below that level. 



It is necessary to push the consideration of the heterogeneous region 

 below the zone of compensation further. To what degree may it be re- 

 garded as a firm, immobile foundation? To what depth does it partake 

 of the character of a mobile layer in which adjustments occur? 



The asthenosphere, according to Barrell, who bases his estimates on 

 the work of Chamberlin and Lunn, corresponds with the outer zone in 

 which the heat conductivity of rocks is lower than in the nucleus, and 

 there is consequently a periodic banking up of heat energy. The depth 

 of this outer zone is about 800 miles.-^ Now, immobility would be pos- 

 sible at constant temperature, but it is impossible in the presence of 

 accumulating heat energy. It has been shown on a previous page that 

 the lithosphere must be intensely vibrant, readily responsive to changes 

 of temperature. There must be corresponding changes of density, and 

 in a heterogeneous mass they would be localized according to conduc- 

 tivity. Similarly, the major effects of fusion and movement of magmas 

 must produce local changes in equilibrium. 



Thus the asthenosphere presents the concept of a deep zone, in which 

 molecular and molar changes of state may introduce unbalanced stresses 

 at any time; and within that zone non-uniform stresses must be very 

 effective in producing molecular or molar movements. It is difficult to 

 escape the conclusion that the asthenosphere is the site of activities which 

 may and do periodically disturb the isostatic balance of overlying masses. 



We are thus led to question the meaning of the term "depth of com- 

 pensation.'^ According to Hayford, it is the depth below sealevel at 

 which the isostatic compensation is complete. If we accept the arbitrary 



29 T. C. Chamberlin and R. D. Salisbury : Manual of geology, vol. i, 1909, p. 566. 



