Vol. X, No. 7.] Isostatic Compensation in India, 279 



[N.S.] 



The probability of the approximate correctness of the 

 results arrived at by me with regard to the form and depth of 

 the Indo-Gangetic depression has recently been most strikingly 

 confirmed by Mr. R. D. Oldham, who had taken up indepen- 

 dently an investigation into the effect on the plumb-line of a 

 depression of the above type filled with alluvium : in a paper 

 published in a recent number of the Geological Magazine, he has 

 pointed out that such a phenomenon is capable of producing 

 the observed anomalies. The rift postulated by Colonel Burrarcl 

 thus becomes superfluous. 



With reference to Colonel Lenox-Conyngham's last criticism 

 on my paper, namely that in dealing with the deflections on 

 the basis of isostatic compensation occurring at a variety of 

 depths, I ought not to have employed the algebraic sums of 

 the residuals in the respective groups, but the sums of their 

 squares, it ought not to be necessary for me to repeat the 

 reason for the course that I adopted, since it was stated clearly 

 not only in my paper, but at greater length in the correspon- 

 dence with Colonel Lenox- Cony ngham, to which I have already 

 referred and which he seems to have overlooked. Not only 

 did I realize, but I stated in my paper, that the sums of the 

 squares would probably give a more accurate result ; I pur- 

 posely refrained f however, from employing that method in 

 order that <my results might be derived in an identical manner, 

 and be therefore directly comparable with those of Major 

 Crosthwait. The sole object that I had in view, so far as my 

 calculations were concerned, was to ascertain how the effects of 

 isostatic compensation occurring at depths other than 113*7 

 km. would compare with those given by Major Crosthwait for 

 that particular depth. Such comparison would not have been 

 legitimate if I had deduced my results by a method different 

 from that employed by him. Even apart from this, the fact 

 that the algebraic sum had been employed in the most recent 

 publication on the subject issued by a body of geodetic experts 

 would seem to show that it had been deliberately employed as 

 being the most suitable method and therefore worthy of adop- 

 tion by others dealing with the same subject. Colonel Lenox- 

 Conyngham's endeavour to justify its use in that particular 

 instance does not appear to me to be very convincing, since 

 he bases such justification on the predominance of sign in the 

 individual groups. If my figures are examined, I think it will 

 be found that the percentage of variation compares not un- 

 favourably with those quoted by Colonel Lenox Cony ngham 

 from Major Crosthwait's paper. Thus in one region, out of 

 four figures, two have a positive sign and two a negative,— 

 thus producing the maximum variation possible, fifty per cent 

 each way. In another region out of 17 figures two have no 

 sign at all, while of the remaining fifteen 20 per cent have one 

 sign and '80 per cent the other. In yet another region the 



