336 Status of the Theory of Isostasy. 



For the United States, Hayford used, as a test of his 

 hypothesis, the ratio of mean residuals to the mean topo- 

 graphic deflections. 35 This is the ratio of column H 

 divided by column D. Such a test is in reality invalid 

 for reasons which have been discussed previously; but 

 for purposes of comparison with the United States this 

 ratio is also given in the present table for India as column 

 J. The values selected from the United States are for 

 the whole of the United States and for groups 5 and 10 

 of the fourteen regions which compose the whole. 



Group 5 embraces the Lake Superior region and is 

 that for which the residuals bear the highest ratio to 

 the topographic deflections for any region in the United 

 States, making the poorest showing for the special 

 hypothesis. Group 10, in southern California, is the 

 region showing the largest topographic deflections and 

 largest residuals of any region in the United States. 



It is seen that for India, exclusive of the Himalayas 

 and the Gangetic trough at their feet, the Hayford hy- 

 pothesis of isostasy gives as satisfactory results, as 

 shown in column J, as for the United States. This 

 agrees with the conclusions of Bowie as based on gravity 

 determinations. 36 



The statement of Crosthwait that "speaking gener- 

 ally it would appear that isostatic conditions are much 

 more nearly realized in America than in India" is not 

 true for India as a whole. The residuals of groups in 

 India removed from the Himalaya are as small as those 

 of groups of equal size in the United States. Crosth- 

 wait 's comparison was poorly based in that he compared 

 individual groups in India with the four quarters of 

 the United States. Furthermore, he did not separate in 

 his statement the problem of the Himalayas and the 

 Indo-Gangetic trough from that of the plateau of India. 

 Instead, then, of using the large residuals adjacent to the 

 Himalayas as an argument that isostasy does not apply 

 well to India, the proper attitude is to ask what it means. 

 Fortunately Oldham has investigated this problem and 

 has gone far toward solving it in a recent comprehensive 

 research. 37 



Oldham simplified the topography by determining the 



35 J. F. Hayford, Supplementary investigation, p. 59, 1910. ^ 



36 William Bowie, Isostasy in India, Jour. Wash. Academy Sciences, 4, 

 245-249 1914. 



"ED Oldham, The structure of the Himalayas, and of the Gangetic plain, 

 as elucidated by geodetic observations in India, Memoirs Geol. Survey India, 

 42, pt. 2, 1917. 



