THE SUPPORT OF THR HIMALAYAS. 109 



increasing excess, as the range is neared, of gravity, which may 

 be interpreted as an increasing defect of compensation. 



Northwards of Mnssooree there are no gravity stations, but 

 in the group to the westward we have the station of Simla, situated 

 about J 3 miles farther into the hills than Mussooree, whether we 

 measure the distance from the main boundary or the outer limit 

 of the hills. At Simla the Bouguer anomaly only has been calcu- 

 lated, which is negative and larger in amount than at Mussooree 

 by -009 dyne; as has been explained, the various corrections re- 

 quired to convert this into the ITayford anomaly would be very 

 much the same at both stations, with the exception of the effect 

 of the compensation of the range itself, and a reference to table 

 No. 11 shows that this effect should be greater at Simla than at 

 Mussooree by somewhere about -035 dyne. From this it results 

 that we should have expected the Bouguer anomaly at Simla to 

 be greater than at Mussooree by not less than about -035 dyne, 

 whereas the excess of negative anomaly is just short of -01 dyne, 

 and it is, consequently, reasonable to conclude that the Havford 

 anomaly at Simla would certainly be positive and larger in amount 

 than at Mussooree, probably somewhere near -j- -08 dyne. 



From this it will be seen that the progressive increase in the 

 defect of compensation, as compared with the hypothesis on which 

 the Harford tables arc based, is repeated in this part of the outer 

 Himalayas, and that the magnitude and rate of increase is not very 

 largely different in the two regions if we take the outer edge of the 

 hills as the starting point for measuring distances. If, on the other 

 hand, we take the position of the main boundary as the zero datum 

 for distance, the anomalies are larger in the western group by close 

 on '05 dyne. Here we have a distinct suggestion that the main 

 boundary, which may be regarded as a dominant feature of geolo- 

 gical structure, is not continued into the region of compensation, 

 but is confined to the outer portion of the crust. 



This suggestion is an important one, and an attempt was made 

 to test it by a detailed examination of the observations in the Dehra 

 Dun district ; the result showed that the apparent discrepancies 

 between the observations were distinctly diminished if the com- 

 pensation was regarded as distributed with reference to the general 

 course of the range, rather than if it was distributed with reference to 

 the sinuosities of the course of the main boundary ; but the result 

 showed that there were also variations in the force of gravity 



r 257 1 



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