THE STRENGTH OF THE EARTH'S CRUST 149 



this article, however, two other tests are applied which indicate 

 that although in some areas compensation does not extend to 166 . 7 

 km. radius, in other areas it extends farther. It is concluded that 

 the United States shows regional departures from isostasy over 

 areas many times larger than Hayford thought to exist, as broad 

 and in some regions probably somewhat broader than the areas of 

 the Nile and Niger deltas, the breadth depending in considerable 

 part upon the magnitude of the loads per unit of surface.^ 



GEODETIC MEASUREMENTS OF ISOSTASY BY HAYFORD AND BOWIE 



Hayford's conclusions from deflections of the vertical. — The posi- 

 tions of many stations over the United States were determined with 

 great accuracy by geodetic measurements from other stations, thus 

 making a closed network. The positions were also determined by 

 astronomic observation. The differences in latitude and longitude 

 between the geodetic and astronomic positions give the observed 

 deflections of the vertical due to the attraction of the surface 

 irregularities and internal heterogeneities of the geoid. To account 

 for these deflections the gravitative attraction upon the plumb-line 

 at each station of all the topography from ocean bottoms to moun- 

 tain tops within 4,126 km. was computed. The influence of the 

 topography alone upon the direction of the vertical is known as the 

 topographic deflection and averages a little over 30". The average 

 of the actually observed deflections are, however, but a fraction of 

 this value. Consequently the excesses of volume represented by 

 continents above oceans, and by plateaus on continents must be 

 very largely balanced and neutralized by corresponding deficiencies 

 of density in the crust beneath, which in turn explains how the 

 larger relief is sustained. This is the theorem of isostasy. Various 

 hypotheses in regard to the magnitude and distribution of these 

 deficiencies in density under the continents, of excesses under the 

 oceans, may be made, and the deflections recomputed on these 

 successive suppositions and compared with the observed deflections. 



'At the recent meeting of the Geological Society of America, December 30, 1913, 

 to January i, 19 14, Professor W. H. Hobbs gave a paper on "A Criticism of the 

 Hayfordian Conception of Isostasy Regarded from the Standpoint of Geology." 

 The writer did not have the pleasure of hearing this paper, but it is clear that 

 Professor Hobbs has attacked independently the same problems as here discussed. 



