124 OLDHAM: THE STKUOTUBE OF THE HIMALAYAS, ETC. 



the very considerable defect of gravity at Monghyr and the lesser 

 defect at Sasaram, which cannot be attributed to the alluvium, 

 but could find an explanation in a depression of the crust into 

 the denser material below, though whether this explanation is 

 valid cannot be established. 



It is outside the alluvial area that the test of the hypothesis 

 must be looked for ; the boundary of the alluvium would not neces- 

 sarily coincide with that of the trough, for south of the alluvium 

 the general level of the surface continues to rise, and in this region 

 we may look for effects to be recognisable, which would be masked 

 by others, of greater magnitude, in the alluvial plain. Now the 

 investigation by Sir S. G. Burrard of the deflection of the. plumb- 

 line in India, published in 1901, l showed that along the northern 

 edge of the peninsular area the deflections were all to the south- 

 wards, and that further south comes a belt in which northerly 

 deflections prevail. His investigation established the conclusion 

 that these facts could only be explained by the existence of a 

 belt of excess of gravity, or as he expressed it a Hidden Range, 

 traversing the Peninsula in a direction approximately parallel to 

 the Himalayan Range, and having its crest directly under the 

 station of Kalianpur. This conclusion has since been supported 

 by the gravity observations, and by Major Crosthwait's deter- 

 mination of the residuals of unexplained deflection at a number 

 of stations in India. The highest positive anomalies of gravity 

 are at Kalianpur and Seoni ; between these stations and the 

 alluvial plain, positive anomalies prevail, but of lesser amount ; and 

 the line of separation between those stations at which Major 

 H. L. Crosthwait obtained a southerly, and those which show a 

 northerly, residual, also runs through these two places and follows 

 almost exactly the course of the " Hidden Range " as indicated 

 by Sir S. G. Burrard in 1901. In the diagrammatic representa- 

 tion, reproduced in fig. 9, of this belt of greater density it is shown 

 as comparatively narrow and steep-sided, and in this form the 

 result would not accord very well with observation, a mass of the 

 form indicated would produce effects distributed very much as 

 shown by the figures in table No. 1, immediately over the crest 

 there would be no deflection, then a gradual increase to a maximum 

 and a gradual dying out again as the distance increased. Actual- 

 ly, however, the observations suggest the existence of local 



» Survey of India, ProijPaper No. 5, Dehra Dun, 1901. 

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