108 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



for some thousands of feet under this station. If the thickness is 

 taken at 10,000 feet and the distance of the station from the bound- 

 ary of the trough at between 2 and 3 miles, the defect in the attrac- 

 tion of gravity would amount to about "05 dyne, or much the same 

 as the excess at Mussooree. It will be shown that the excess of 

 gravity, which may be attributed to the want of adjustment of the 

 compensation, decreases in the stations southwards of Dehra Dun, 

 and so we may reasonably conclude that it will be less at that 

 station than at Mussooree, but if the trough is 10,000 feet deep under 

 Dehra Dun, there would need to be an excess of gravity apart from 

 the effect of the trough of not less than the anomaly at Mussooree, 

 and so again we find that the depth of the trough under the Dehra 

 Dun is somewhat under 10,000 feet. 



To the southwards of Dehra Dun are some gravity stations at 

 which the Hayford anomaly has not been calculated, but can be 

 estimated approximately by comparison of the corrections at the 

 stations already considered, with those in the plains to the south. 

 Accepting the figures given on p. 92, we find that at Asarori 

 and Hardwar there is an anomaly of — -1 dyne ; as the effect of the 

 trough would amount to at least — -04 to — -05 dyne, there is left 

 an excess of gravity of from -f- "03 to -f -04 dyne at these stations. 

 At Mohan the anomaly, exclusive of the effect of the trough, is 

 about — -02, and the effect of the trough will be much the same as 

 at Hardwar, leaving an excess of gravity of about -f- -02 to -f 03 

 dyne. At Roorkee, the Hayford anomaly has been calculated as 

 — -043 dyne, and if this is directly interpreted in terms of the depth 

 of alluvium necessary to produce the same effect, it represents 

 a thickness of about 6,500 feet, but if interpreted in the terms of 

 difference from Dehra Dun, indicates a depth of about 6,000 feet 

 greater than at the latter station. In the last chapter a figure 

 of 13,000 feet was indicated as the approximate depth indicated 

 by the geodetic data, a figure which agrees very well with that 

 indicated by deduction from the geological structure, and a com- 

 parison of this figure with those given in the last sentence suggests 

 that the defect of compensation, found in the stations to the north, 

 still exists under Roorkee, though the effect is reduced in amount 

 to not more than + -02 dyne. Here, however, we have reached 

 a region where too many corrections of unknown amount have to 

 be applied for the result to be of any real value, but the stations 

 to the northward, in the Dehra Dun district, indicate a gradually 



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