66 OLDHAM: THE STRUCTURE OF THE HIM A L A V AS, ETC. 



A list of these stations is given in table No. 19 (page 65) with 

 the addition of four stations, ranged along the northern fringe of the 

 alluvium to the eastwards of the meridional scries. It is in no case 

 possible to measure the exact distance of any of these stations from 

 the main boundary, as this runs through Nepal territory to the 

 north of this section, but the distance from the outer edge of the 

 hills can be determined with suflieient accuracy, and a comparison 

 of the section at the western end of Nepal with that along the. 

 road to Khatmandu, shows that the width of the Siwalik tract is 

 probably about 20 miles, so that the main boundary may be taken 

 as lying at that distance from the outer edge of the hills and, where 

 it needs to be taken into consideration, this must be added to the 

 distance of each station from the outer edge of the hills as given 

 in the table No. 19. In this table are also given the distances 

 from the southern boundary of the alluvium, the figures in each 

 case being approximate and measured to the boundaries of the 

 alluvium as drawn on the general geological map of India on the 

 scale of 32 miles to the inch. 



The deflection, actually observed at each station, is given, to 

 the nearest whole second, in the last column of the table, and the 

 first thing to be noticed is the presence of a considerable southerly 

 deflection at the stations beyond the alluvium to the south. The 

 distances of these stations from the boundary are too great for 

 the deflections to be attributable to the effect of the alluvial trough, 

 and we may look for their cause in the " hidden range " or belt 

 of underground excess of density which has been found to exist 

 in the northern part of the Peninsula. 



Turning to the stations on the alluvium, and comparing the 

 observed values with the calculated deflections given in tables 14 

 to 16, we see that, so far as the southern half of the section is con- 

 cerned, they indicate a trough deepening steadily from south to 

 north at about 130 ft. to the mile, and that this slope is continuous 

 for over 100 miles from the southern edge, so that in this way 

 we reach an estimated depth of over 13,000 and probably about 

 15,000 feet. The northern part of the section gave more trouble, 

 for here the effect of the Himalayas, which is negligible at the 

 southern stations, becomes considerable. As it was impossible to 

 calculate the effect of the actual topography at each station it 

 seemed best to assume that the effect would not be very different 

 from that of the Imuginary llangc, allowing for Hayford compen- 



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