82 OLDHAM: THE STRUCTURE OK THE HIMALAYAS, ETC. 



a Bouguer anomaly of — -031, and a Hayford of — 024 dyne, and, 

 as it is difficult to believe that there can be a thickness of over 

 4,000 feet of alluvium under this station, we must fall back on the 

 supposition that the anomaly is due to a more deep-seated defi- 

 ciency of density. A similar, though smaller, defect of density 

 at the station of Sasaram, included in table No. 23, suggests that 

 in both cases the anomaly may be due to a deep-seated defect of 

 density in the rocks below the alluvium. 



However this may" be, the solitary exception, if it be an excep- 

 tion, does not affect the general conclusions which have been drawn 

 from the gravity and latitude observations, conclusions which are 

 besides in agreement with the inferences from geological obser- 

 vations. It is, of course, possible that the results might be due 

 to a defect of density in, or below, the rocky crust of the earth, 

 of an amount and variation similar to that which has been attri- 

 buted to variations in the depth of the alluvium, but this explana- 

 tion is not probable, and is inapplicable in the case of the deflec- 

 tions at stations near the main boundary, for these could not be 

 explained by any deep-seated cause, but require a defect in density 

 immediately below the surface, such as would necessarily result 

 from the known facts of geological structure. We may take it, 

 therefore, that the results of geodetic observation at stations on 

 the Gangetic alluvium indicate, firstly, that the southern margin 

 of the Gangetic trough is very much as marked on the map, PI. 12 ; 

 secondly, that a large area of alluvium east of the Aravallis does 

 not, properly speaking, belong to the trough, but is merely a thin 

 covering of alluvium laid down upon, and obliterating the un- 

 evenness of, an irregular land surface ; thirdly, that the alluvium 

 east of the Rajmahal Hills, stretching southwards to the Gangetic 

 Delta and eastwards into the valleys of the Brahmaputra and 

 Barak, also lies outside the limits of the Gangetic trough, and is 

 formed by a comparatively thin covering of alluvium, whose thick- 

 ness may be measured by hundreds, instead of thousands of feet, 

 and, fourthly, that the Gangetic trough proper, reaches a depth 

 of 15,000 to 20,000 feet towards its northern edge, and that its 

 floor has a fairly regular upward slope to the southern margin. 



The geodetic stations in the Punjab plains are fewer in number 

 and more scattered, than those on the Gangetic alluvium, yet, 

 interpreted in the light of the latter, they give some interesting 



[ 230 j 



