92 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



compensation will not be great, come the three stations of Asarori, 

 Hardwar, and Mohan, all situated on the line of the Siwalik Hills, 

 and all indicating a depth of about 10,000 feet. Southwards of 

 these again come the two stations in the alluvium indicating a depth 

 of 13,000 and 12,000 feet. Here again it is not the exact figures which 

 are important, although it has been shown that they are probably of 

 much the same order of magnitude as the actual depths, but the 

 very definite indication of an increase in depth of the trough to the 

 south of the edge of the Siwalik Hills. The amount of this difference 

 is 3,000 feet as between the stations in the Siwaliks and Roorkee, 

 but Roorkee is separated by about 15 miles of plain from the Siwalik 

 Hills, and the stations to the southward indicate a progressive 

 decrease in depth at the rate of about 250 feet per mile for some 

 forty miles from Roorkee. If this average slope continues north- 

 wards from Roorkee towards the hills, the actual rise in the floor of 

 the trough may well amount to the 5,000 feet assumed when dealing 

 with the deflection of the plumb-line. 



The gravity observations may also be treated in another manner. 

 At the four stations of Rajpur, Dehra Dun, Roorkee, and Kaliana, 

 we have "both the Bouguer and Hayford anomalies, from which it is 

 easy to obtain the correction from the one to the other at those 

 stations. If, then, these corrections at the four stations are plotted 

 on squared paper, the stations being ranged according to their 

 distances from the main boundary, a curved lino can be drawn 

 through the four points which will approximately indicate the correction 

 which would be applicable to a station at some other distance from the 

 main boundary, and by applying this correction to the published 

 Bouguer anomalies, we can get an approximate value for the Hayford 

 anomaly, which should be correct to the first two places of decimals. 

 The values obtained by this method are given below, where an 

 asterisk means that the anomaly is an estimated one ; the figures 

 are : — ■ 



Distance. Anomaly. 



+ -022 

 -f 04' 

 + 033 

 + -01* 



— 01* 



— •01* 



— 02* 

 _-043 



— 04* 



[ 240 } 



Rajpur 



miles 



Kalsi 



„ 



Dehra Dun 



• 2 „ 



Fatehpur 



6 „ 



Hardwar 



"' >, 



Asarori 



9 „ 



Mohan 



U „ 



Roorkee 



25 „ 



tfojli 



■ 38 „ 



