78 OLDHAM: THE STRUCTURE OE THE HIMALAYAS, ETC. 



proper, so far as it can be inferred from surface observations, ex- 

 cluding the spreads of, presumably shallow, alluvium with inliers of 

 rock rising from it, which belong more properly to the rock area 

 of the Peninsula than to the Gangetic alluvium. The third column 

 of the table gives fehe Bouguer anomaly, or the difference between 

 the observed value of gravity at the station and the theoretical 

 value, after allowing for the effect of latitude, altitude, and the 

 attraction of the mass above sea level, reckoned as rock of average 

 density. 



This anomaly is negative at every station but two, where its 

 positive value is so small as to be practically non-existent ; in other 

 words, there is everywhere an apparent defect of gravity. At 

 the stations nearer to the Himalayas a part of this defect is due 

 to the compensation of the range, and in the case of these stations 

 the fourth column gives the amount which this compensation 

 would be in the case of the Imaginary Range, a figure which is 

 slightly, but not materially, less than the compensation of the 

 actual range as calculated by Mr. Hayford's tables. After allowing 

 for this there still remains a defect which may be due to various 

 causes, of which one is the defect of density of the Gangetic allu- 

 vium as compared with an equal bulk of average rock, and in the 

 fifth column is the depth of alluvium, to the nearest 500 feet, ' 

 which would be equivalent to the anomaly of gravity at the station. 



It must not be supposed that these figures necessarily represent 

 the actual depth of the alluvium, for they might be modified in 

 various ways ; the adoption of the later formula for the variation 

 of gravity with latitude would increase them by about 3,500 feet ; 

 the newer densities would introduce only a very slight change at 

 any of the stations, but the effect of distant topography, beyond 

 a radius of 100 miles, and its compensation, which is not taken into 

 consideration, introduces a further correction to the depth of the 

 alluvium. The amount of this last correction has been published 

 in the case of only one of the stations, Arrah, where it is -028 dyne, 

 and at Dehra Dun, to the north of the group included in the table, 

 it is -057 dyne : as the effect is in both cases negative it would reduce 

 the numerical value of the anomaly and consequently the appa- 

 rent depth of the alluvium by from 4,000 to 8,000 feet. All these 

 corrections would, however, affect the stations to much the same 

 extent and, though they would alter the absolute value of the 

 inferred depth of the alluvium, would have little effect on the 



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