312 JOSEPH BARRELL 



this question is settled by geodesists, equal weight should perhaps 

 not be attached to the figures given for the departures from isostasy 

 shown over the Tonga plateau and Tonga deep. Neither is the 

 area of these departures known, though the areas of the plateau 

 and deep are large. These regions are seen, however, to indicate 

 considerably higher departures from isostasy than the measure- 

 ments determined from the deltas of the Nile and Niger. The 

 latter, therefore, perhaps do not measure the full strength of the 

 crust. 



Major H. L. Crosthwait has applied Hayford's methods to the 

 investigation of isostasy in India.^ The residuals of the deflections 

 of the vertical serve as a measure of the degree of compensation 

 existing in the United States as compared in India and are as fol- 

 lows: 



United States of America 



Group S.E., mean residual — o f 74 



Group N.E., mean residual — i . 04 



Group Central, mean residual — i . 66 



Group W., mean residual — 4.02 



India 



Region No. i, Himalaya Mountains, mean residual — 16 



Region No. 2, Plains at foot of Himalaya Mts., mean residual — 2 



Region No. 3, N.E., mean residual + 8 



Region No. 4, Central, mean residual + 5 



Region No. 5, N.W., mean residual + 4 



Region No. 7, W., mean residual — 3 



Region No. 8, E., mean residual — 2 



Region No. 9, S., mean residual + i 



It is seen that the residuals average several times as great in 

 India as in the United States, which leads him to conclude that 

 "Speaking generally it would appear that isostatic conditions are 

 much more nearly realized in America than in India, i.e., if we 

 are to take the smallness of the residuals as an indication of the 

 completeness of isostatic compensation."^ Colonel Burrard, utiliz- 

 ing the Hayfordian computations, points out the existence of zones 



' Professional Paper No. 13, Survey of India, 191 2. 

 ^ Op. ciL, p. 4. 



