76 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



of the deep trough sweeps across, under the alluvium of the 

 Ganges and Brahmaputra rivers, in an easterly or north-easterly 

 direction from the point where its course ceases to be denned by 

 the boundary between rock and alluvium. Whether the trough 

 extends up the valley of the Brahmaputra cannot at present be 

 decided; the geological evidence of the rocky hills in the alluvium, 

 and the structural analogy which exists between the Assam Hills' 

 and the Salt Range, at opposite ends of the Himalayas, both sug- 

 gest that the deep trough does not extend up the valley of the 

 Brahmaputra, and this conclusion is to a certain extent borne 

 out by the easterly deflection of the plumb-line at Jalpaiguri. 



At this station Major Crosthwait's calculations show that the 

 effect of the visible topography and its compensation should pro- 

 duce no deflection in either direction, yet observation shows that 

 there is an easterly deflection amounting to 18" or 13", according 

 to the Everest and the Bessel-Clarke spheroids, respectively. 

 As this deflection is not due to visible topography we must 

 look to some underground cause, of which a very probable 

 one is to be found in the fact that the station lies near 

 the eastern limit of the Gangetic trough, if this is presumed not 

 to extend up the Brahmaputra Valley. In this case there would 

 be the whole of the trough to the west of the station, unbalanced 

 by any similar extension on the east, and so an easterly deflection 

 would be produced. The magnitude of this deflection is greater than 

 anything met with in the southern part of the trough, further west, 

 and indicates an upward slope, of the bed of the trough, which 

 may amount to as much as 300 to 400 feet per mile, or about 4° of 

 arc, if the whole of the deflection is due to the effect of the trough. 



The gravity observations in the alluvial plains have been dealt 

 with by Dr. H. H. Hayden, 1 who showed that they indicated a gra- 

 dual shallowing of the trough in a southward direction, but it 

 will be well to review the more complete evidence, which is now 

 available. In table No. 23 (page 77) a list of the gravity 

 stations in the region of the Gangetic trough is given, arranged in 

 three natural groups ; the first a series ranged from north to°south, 

 along the 78° meridian ; the second, a more extended group, covers 

 the central portion of the trough, where it reaches its maximum 

 development; and the third ranged along the 88° meridian. The 



1 Rec, Geol. Svrv. Ind., XLIII, 163-167 (1913). 

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