THE SUPPORT OF THE HIMALAYAS. 107 



between the result of the gravity observations and of the deflection 

 of the plumb-line as can be expected, and the want of adjustment 

 between topography and compensation in this part of the range 

 may be accepted as a fact, the consideration of its interpretation 

 and origin being deferred for the present. 



In the western group we have no gravity determinations, of 

 which the Hayford anomaly has been calculated, further into the 

 hills than Mussooree, three miles in from the main boundary, and 

 the same distance in from the outer edge of the Himalayas proper. 

 At this station the Hayford anomaly amounts to -f -049 dyne ; at 

 Kajpur, close to the main boundary, the same anomaly was found to 

 amount to only + -022 dyne, a remarkable difference to find in so 

 short a horizontal distance. Part of this difference is the result of 

 the method of calculation, combined with the fact that Mussooree is 

 situated on the crest, and Rajpur at the foot, of a steep-sided hill, 

 with a difference of level amounting to over 3,500 feet. In the 

 method of calculation adopted, each separate small compartment is 

 supposed to be separately compensated, but it is highly improbable 

 that the compensation can vary as rapidly as the topography in a 

 case like this, and if it varied more slowly the amount would not be 

 largely different in the near-by compartments at each station ; 

 the result being that the actual calculation makes the effect of 

 compensation too great at Mussooree and too small at Rajpur, thus 

 increasing the difference between the anomalies at the two stations. 

 Though part of the difference may be explained away in this manner, 

 it is insufficient to account for more than a part, and probably a 

 small part, and so we are driven to find another explanation, which 

 is provided by the defect of density in the Siwalik rocks. If the 

 trough in which they lie is supposed to be 10,000 feet deep, it would 

 produce a difference of about — -03 dyne at Kajpur as compared with 

 Mussooree, and about — *02 dyne at Dehra Dun as compared with 

 Raj pur, or about the same differences are found in the calculated 

 anomalies, which take no cognizance of the effect of the trough 

 We may conclude, therefore, that the actual excess of gravity at 

 Mussooree and Raj pur is much the same and, interpreted as a defect 

 of the compensation, appropriate to the averaged topography of the 

 region, amounts to something less than "Of) dyne. 



At Dehra Dun the anomaly is very small, but at this station a 

 negative anomaly should have been expected, on account of the 

 effect of the lesser density of the Siwalik rocks, which certainly extend 



[ 255 J 



