304 H. O. WOOD SOME CONSIDERATIONS TOUCHING ON ISOSTASY 



units of small area, or conditions affecting such units. And earlier opin- 

 ion was distinctly adverse to this. 



Geodetic Establishment of isostatic Equilibrium 



On the other hand, recent geodetic work, especially that conducted by 

 Hayford 5 and Bowie, 6 of the United States Coast and Geodetic Survey, 

 and following them by Burrard 7 and his colleagues, of the Trigonomet- 

 rical Survey of India, has gone far toward establishing it as a reality, by 

 a preliminary treatment and least-square adjustment of considerable 

 bodies of refined physical measurements, that the outer shell of the earth 

 above a depth of about 100 kilometers departs only very slightly from a 

 condition of flotational equilibrium among its parts. Even when prisms 

 of the shell of small area are dealt with, the excess or defect of mass 

 found in this way is small, while for prisms of large area the excess or 

 defect of mass thus found is practically negligible — less than two-tenths 

 of 1 per cent of the mass of the prism for the United States. So com- 

 plete and so intimate an adjustment is thus indicated that Hayford ven- 

 tured to predict that "future investigations will show that the maximum 

 horizontal extent which a topographic feature (that is, an excess or defect 

 of mass in a prism — Wood) may have and still escape compensation is 

 between one square mile and one square degree." 



As stated above, it can not be said that geologists in general have been 

 led by geologic evidence to expect so intimate and so completely effective 

 isostatic adjustment. Objections to it have been pointed out. Even if 

 Hayford's limiting estimates of area were to be increased considerably, 

 these objections would hardly be met thereby. 



Implications of complete Isostasy 



Questions have been raised as to the assumptions made and the treat- 

 ment and mathematical adjustment employed by the geodesists in dis- 

 cussing their admittedly precise physical measurements. Consequently 

 their conclusions have been questioned. This is not a matter for geolo- 

 gists, as such, to discuss. At present the objections which have been 

 raised do not seem to be sustained. And, if the assumptions and methods 

 are valid, there can be little doubt that geodesists have established the 

 fact of isostatic adjustment, certainly over large regions of the earth at 

 the present time. 



The geodetic findings imply — nay, require — that the earth shell above 

 the level of isostatic equilibrium must be very weak — at any rate, in 

 offering resistance to stresses exerted in the vertical direction (the pres- 



