THE UNDERGROUND FORM OF FLOOR OF GANGETIC TROUGH. 81 



in its southern half, runs close to the margin of the trough, which 

 here has a course of about N.N.W. — S.S.E., and in its northern 

 half suggests a depth of about 15,000 feet at the outer edge of the 

 Siwalik hills— the northerly continuation of this section across the 

 Dehra Dun being dealt with further on. The central group, 

 situated where the trough attains its maximum breadth, indicates 

 that the maximum depth may reach 20,000 feet or more. The 

 eastern group indicates a depth of about 15,000 feet at the northern 

 boundary, and also the existence of a rock barrier, covered by no 

 great depth of alluvium, connecting the Peninsular area with the 

 Assam Range. This last conclusion is in agreement with the 

 deduction which has already been drawn from the deflection of 

 the plumb-line, and is of interest as showing that the broad ex- 

 panse of alluvium, which stretches southwards to the Gangetic 

 delta, forms no part of the Gangetic trough. 



A confirmation of this deduction is to be found in the gravity 

 observations at the two stations of Kisnapur and Chatra. These 

 are situated on the alluvium, but, in spite of this, both have posi- 

 tive anomalies, the Bouguer being + 033 and -f -009 dyne, and 

 the Hayford -f -039 and -f -005 dyne, respectively. The high 

 positive anomaly at Kisnapur is evidently the result of a deep- 

 seated excess of density in the rock underlying the alluvium, but 

 its magnitude, and the smaller positive anomaly at Chatra, show 

 that the alluvium cannot have any great thickness, comparable 

 to that in the Gangetic trough, for if there were any great thick- 

 ness of alluvium the negative effect of the defect in density would 

 more largely neutralise the deep-seated excess of density in one 

 case, and in the other would make the anomaly negative, instead 

 of positive. We may, therefore, reasonably conclude that the 

 alluvium, spread over the gap between the north-east corner of 

 the Peninsular area and the hills of Assam, is of no great thick- 

 ness, and forms no part of the Gangetic trough, in the sense in 

 which these words are used here, although it has to be coloured 

 the same as the plains of Upper India on a map showing the surface 

 geology. 



There remains one station in the alluvial plain which requires 

 separate notice, as the results are somewhat anomalous. This is 

 the station of Monghyr, near the southern margin of the alluvium, 

 where the width of the plain has diminished to ninety miles. 

 Though situated close to the southern edge of the alluvium it <nves 



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