THE UNDERGROUND EORM OE ELOOR OE GANGETIC TROUGH. 91 



table No. 27 bearing the same proportion to 15,000 feet as the un- 

 explained anomaly to the deficiency of gravitation which should be met 

 with at a station similarly situated on a trough 15,000 feet in depth. 

 The first point to be noticed in the table is that the depth of the 

 trough at Raj pur is given as 15,000 feet, which happens to be exactly 

 the figure assumed at the outset as somewhere near the actual 

 throw of the main boundary fault, but, as has been explained, no 

 great importance can be attached to the precise figure. The second 

 point to be noticed is that the western stations of Kalsi and Fatehpur 

 give much smaller depths of the trough and, at first sight, seem 

 to indicate that the throw of the main boundary fault in this section 

 is only about one half as great as on the Rajpur-Dehra Dun section. 

 The correctness of this conclusion is, however, open to doubt, owing 

 to the unknown effect of the break in the general course of the main 

 boundary just east of Raj pur, and this doubt is confirmed by a 

 consideration of the Hayford anomalies. These have positive values, 

 of + "003 at Dehra Dun and -f- -022 at Raj pur. thus following the 

 general rule that the Hayford anomaly has a positive value as com- 

 pared with the Bouguer, but the amount of the difference is greater 

 than at stations further removed from the Himalayas ; and, more- 

 over, the anomaly is greater at Raj pur than at Dehra Dun. There 

 are two possible explanations of these differences, between the Hay- 

 ford anomalies at Rajpur and Dehra Dun and between the Bouguer 

 anomalies at these and stations further west ; they may be due, 

 either to a variation in the depth, and consequent effect, of the 

 trough, or to a difference between the real and the calculated effect of 

 the compensation of the range, for all other changes, introduced by 

 the difference in the method of calculation, as well as by the effect of 

 any cause not considered in the calculations, would affect both stations 

 in exactly, or very nearly exactly, the same degree and direction. 



From this it appears that these stations, close to the main 

 boundary, cannot be used with any degree of safety in determining 

 the form of the trough, or in other words, they belong more properly 

 to the region of the range and will be more profitably dealt with in 

 that connexion. It also follows that the gravitation observations 

 close to the main boundary cannot bo used to confirm or qualify 

 the results obtained from the deflections. 



Southwards of the stations just considered, and now at a sufficient 



distance from the edge of the range proper to make it probable that 



the difference between the actual and the calculated effect of the 



[ 239 1 



