THE UNDERGROUND FORM OF FLOOR OF GANGETIC TROUGH. 95 



Fig. 7.— Generalised cross-section of the Gangetic trough. This does not 

 represent any individual cross-section but is a diagrammatical representation of 

 the general type ; to the left is the Siwalik region with its successive boundary 

 faults, which now forms part of the mountain system of disturbance; to the 

 right is the alluvial trough proper, the floor of which at first slopes downwards 

 to the point of maximum depth, and then gradually upwards to the southern 

 limit of the alluvium. 



Leaving the hills, the section enters the area of the alluvial plain 

 and there is an increase in depth of deposit ; beyond this the section 

 becomes uncertain for a while and there are two possibilities, one 

 that the floor of the trough slopes upwards from a maximum 

 depth at the edge of the hills, the other that the trough gradually 

 increases in depth for a while before the upward slope of the floor 

 commences, as is indicated in the figure. In either case the greater 

 part of the width is occupied by a sloping floor, rising to the south- 

 wards and ending in a rock area, rising above the level of the plain 

 in some sections, and in others covered by a layer of alluvium. 



The position of the southern boundary has been referred to when 

 dealing with the different groups of observations. At the eastern 

 end the boundary seems to bend round to the northwards, and the 

 trough to terminate where the Assam range impinges on the boundary 

 of the Himalayas. The next locality where the boundary of the 

 deep trough can be fixed is to the south of Jalpaiguri, where it 

 evidently runs near to the stations of Chanduria and Ramchandpur ; 

 the exact position here is doubtful as the deflection suggests that the 

 boundary lies to the southward of, and the anomaly of gravity that 

 it lies very nearly under, or a little to the northward of, Ramchand- 

 pur. The distinction between the deep trough and the shallow 

 covering of alluvium must in any case be an indefinite one and 

 cannot be defined with accuracy, but the trough here has a width 

 of certainly 80, and possibly over 100, miles. 



In a westward direction the southern boundary of the trough is 

 fixed by the boundary of rock and alluvium at Monghyr and thence 

 sweeps across to Sasaram, the stretch of alluvium to the southward 

 of this line, with rock islands rising from it, being evidently of only 

 shallow depth. From Sasaram westwards the boundary of the 



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