80 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



the figures given on page 82 which show that it would amount 

 to about -015 dyne at the southern margin, and to about -040 at 

 thirty or forty miles from the northern margin, or more where 

 the trough has a greater width than 100 miles or a greater maximum 

 depth than 15,000 feet. The former of these figures would neu- 

 tralise the effect of about 2,000 feet of alluvium, the latter about 

 5,500 feet to perhaps 7,000 in the central portion of the trough ; 

 and so, the difference between the northern and the southern 

 stations, or the apparent depth at the northern stations of each 

 group, would be increased by about 3,000 to 5,000 feet. 



From this it will be seen that the modifications introduced by 

 these two considerations practically neutralise each other and the 

 figures in the table remain as the closest approximation to the 

 actual depth of the alluvium which can be attained by this method. 



In the last column of the table No. 23, another series of figures 

 is given, based on the Hayford anomalies, where these are available. 

 The thickness given here is not directly deduced from the anoma- 

 lies, because these are positive at several stations, indicating a 

 negative thickness of the alluvium, which is impossible. The posi- 

 tive anomaly reaches its maximum at Agra, where it is equivalent 

 to the effect of about 2,500 feet of alluvium, and if the thickness 

 at Agra is made equal to 500 feet to bring it into accord with the 

 depth of the alluvium, which is known to be 480 feet at that place, 

 a correction of 3,000 feet must be made to the thickness deduced 

 at the other stations, assuming that the difference in the anomaly 

 is due to a difference in thickness of the alluvium. 



In this way the figures in the last column were obtained, and 

 it will be seen that they follow the same general course as those 

 in the preceding column, but indicate a lesser depth at the northern 

 stations. Here, however, it must be remembered that the Hay- 

 ford anomaly includes the effect of distant topography and its 

 compensation, and if allowance is made for this, the figures in the 

 last two columns would come into very fair agreement with each 

 other, as close, probably, as can be expected. In both columns 

 there are some departures from a regular decrease in depth as the 

 southern margin of the alluvium is approached, departures which 

 may be due to local variations in the force of gravity, quite uncon- 

 nected with the trough, and also to irregularities in the form of 

 its floor, which may be considerable when expressed in feet, though 

 subordinate to the general slope of the floor. The western group, 



[ 228 ] 



