THE UNDERGROUND FORM OF FLOOR OF GANGETTC TROUGH. 79 



differences, or at any rate the differences between any two adja- 

 cent stations. 



Confining attention to these differences alone, it will be seen 

 that in the first group there is a steady decrease in depth as the 

 distance from the northern boundary increases and the southern 

 boundary is neared ; the second group repeats the same feature, 

 as does the third, though here the position of the southern bound- 

 ary can only be inferred from the geological structure of the rock 

 area on either side of the alluvium which stretches southwards 

 to the delta of the Ganges, and from the geodetic observations 

 themselves. 



The gravity observations, then, agree with the observations 

 of the deflection of the plumb-line in bearing out the conclusions, 

 which had been drawn from geological examination, as to the 

 general upward slope of the floor of the Gangetic trough from north 

 to south ; and the fact that the gravity observations indicate a 

 thickness of less than 500 feet at stations near the southern margin, 

 where the thickness of the alluvium is either known, or may be 

 presumed, to be small, suggests that the various corrections, which 

 have been referred to above, neutralise each other, so that the 

 figures given in the table may be regarded not merely as compara- 

 tive, but as not far from the actual depth, or at least of about the 

 same order of magnitude as it. There are, however, two consi- 

 derations which may introduce a modification of this conclusion. 



The first of these is the effect of distant topography and its 

 compensation. As has been mentioned, this is greater by about 

 •030 dyne at Dehra Dun, just north of the stations included in the 

 table, than at Arrah, and as the difference is probably very largely 

 due to the greater proximity of Dehra Dun to the Himalayas, it 

 is also probably greatest at the northern stations of each group, 

 and decreases progressively in the southern. As the effect of 

 this correction would be to decrease the apparent thickness of 

 the alluvium, it is evident that the variation in its amount would 

 decrease the difference between the apparent depths at the north- 

 ern and the southern stations ; and, as the thickness at the southern 

 edge must necessarily be nothing, the result would be an apparent 

 decrease in the depth at the northern stations of each group by 

 some 3,000 to 4,000 feet. 



Secondly, we have to consider the effect of a separate compen- 

 sation of the trough. The amount of this effect is indicated by 



[ 227 J 



g2 



