88 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



take place with increasing distance from the main boundary, and 

 the first supposition which was investigated was that this cause is 

 the attraction of the mass of the Siwalik plateau, above the general 

 level of the plain. The figures given in table No. 6 show that this 

 would produce a southerly deflection of about .14" at the northern 

 boundary, decreasing to zero in the centre, about coincident with 

 the position of the Dehra Dun E. Base station, and a northerly 

 deflection in the southern half, increasing to about 14" at the southern 

 edge. This effect was added to that of the Imaginary Range and 

 of a trough of uniform depth of 15,000 feet ; and the sum, converted 

 into the meridian, by allowing for the departure of the course of 

 the range from due east to west in this region, is given in column I 

 of the table. 



It will be seen that the figures arc in very fair accord with the 

 result of observation, except in the case of the two stations at Dehra 

 Dun, but here the uncertainty as to the precise course of the main 

 boundary, under the surface gravels north-eastwards of the stations, 

 introduces so great an uncertainty into the calculation of the 

 deflections to be expected at them, that these two stations might 

 well have been left out of account in this connexion. Apart from 

 this, the hypothesis not only gives about the same difference between 

 the deflections at Raj pur and at the Dehra Dun E. Base station, 

 and the group beyond it in the .Siwalik hills, but provides for the 

 same uniformity of deflection, at all distances between 10 to 16 

 miles from the main boundary, which is exhibited by the actual 

 deflections ; and this uniformity would not be seriously disturbed 

 by a difference of anything under 5,000 feet in the assumed depth 

 of the trough, though the actual figures, and the difference between 

 the calculated values at Raj pur and Hatni, would be somewhat 

 increased or diminished as the case might be. An assumption that 

 the depth of the trough decreased continuously with increased distance 

 from the main boundary would seriously disturb this uniformity, for 

 it would introduce a rate of decrease in the northerly deflections 

 which would more than counterbalance the effect of the Siwalik 

 plateau, and require a distinctly greater northerly deflection at the 

 Dehra E. Base station than at those further removed from the main 

 boundary. From this we might conclude that the depth of the 

 trough at Raj pur is somewhere about 15,000 feet and that this depth 

 is maintained in a southerly direction to a distance of 30 or 40 

 miles from the boundary, before the shallowing of the trough begins. 



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