74 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



be small ; it may be felt, to the extent of a couple of seconds or so, 

 at Mednipur, Nuaon, and possibly Dubauli, but at the stations 

 further north it can have no appreciable effect, and the observed 

 deflections may be ascribed principally, if not wholly, to the effect 



of the Gangetic trough. 



The easternmost group of latitude stations is ranged near 88° 

 of longitude ; in table No. 22 a list of those which lie in or near 



Table 22. — Latitude Stations near 88° Longitude. 



Station. 



Distance from 



Main Boundary 



(in miles). 



Calculated 

 Deflections. 



Observed 

 Deflections. 



♦ 









Kurseong 



4 N. 



— 44 



— 46 



Siliguri .... 



12 S. 



— lit 



— 18 



Jalpaiguri 



30 „ 



- 2 : 



— 2 



Lohagara 



52 „ 



. +. j 



+ <3 



Chanduria 



76 „ 



+ 7 



+ 8 



('haraldanga . 



120 „ 



+ 1 



+ 6 



Madlnipur 







+ 8 



the alluvial plain 18 given, ranged from north to south. In decid- 

 ing on the assumption to be made in calculating the deflec- 

 tions which should be expected we encounter the difficulty that, 

 owing to the absence of a belt of foot-hills corresponding to the 

 Siwaliks, it is impossible to form any direct estimate of the depth 

 of the trough next the hills. The gravity determinations, as will 

 be seen, indicate that this is not materially different from the 

 depth in the Dehra Dun region, and so we may take it at about 

 15,000 feet ; the width is less easily determinable, as, though the 

 alluvium extends continuously to the sea-face of the Gangetic 

 delta, the real boundary of the trough probably lies on a line con- 

 necting the northern end of the Rajmahal Hills with the western 

 end of the Garo Hills. Here, as has already been mentioned, there 

 is some geological ground for supposing the existence of a ridge 

 of rock, covered by a comparatively shallow layer of alluvium, 

 the crest of which might lie about 20 miles north of Charaldanga, 



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