THE UNDERGROUND FORM OF FLOOR OF GANGETIO TROUGH. 85 



about the same distance from the main boundary as Khimuana, 

 seem to lie on a comparatively thin covering of alluvium, which 

 is 8,500 feet thick at Ludhiana and 23,000 feet at Pathankot. 

 From these figures we may conclude that the trough is, on this 

 section, less than 100 miles broad, but has a depth which is com- 

 parable with, and possibly quite as great as, that of the much 

 broader trough in the Gangetic region ; further, if we take the 

 stations of Mian Mir and Pathankot, the gravity observations give 

 a mean slope of the floor of the trough of about 250 feet per mile, 

 or just about the same as is indicated by the deflection at Amrit- 

 sar, a station which lies between the other two, and close to where 

 the southern edge of the depression seems to lie. 



The conclusions drawn from the Bouguer anomalies require 

 some modification when the Hayford anomalies are used. At 

 Mian Mir and Pathankot these are -f 040 and — '077, respectively, 

 giving a difference of -117 dyne, equivalent to the effect of about 

 17,500 feet of alluvium. The large positive anomaly at Mian 

 Mir precludes this interpretation and the actual anomaly at Pathan- 

 kot represents a depth of only 11,500 feet, if the anomaly is solelv 

 due to this cause. The positive anomaly at Mian Mir shows that 

 the alluvium cannot have any great thickness here, but the anom- 

 aly itself must be due to an excess of density in the rocks below 

 the alluvium, and may be deep-seated enough to account for part 

 of the high southerly deflection at Amritsar ; some such cause Is 

 necessary if the depth of alluvium at Pathankot, deduced from 

 the Hayford anomaly is approximately correct, for this would give 

 a mean slope of only»about 120 feet per mile to the floor of the 

 trough, and produce a deflection of not more than 6" to 7" 

 away from the range. 1 



The geodetic observations in the Punjab, like those further 

 east, give different numerical results according to the way they 

 are dealt with, but, in spite of this difference in the dimensions 

 of the trough, they agree as to its general form and show that the 

 depression, now filled with alluvium, which has been traced along 

 the southern edge of the Himalayas from near the 89° meridian, 

 continues westwards at least as far as 74° ; and that it there main- 

 tains the same general character of deepening regularly from the 



1 At this station the adoption of the Bcssel-Clarke spheroid would increase the 

 southerly deflection by about 3" (S. G. Burrard, Phil. Trans., Series A, CCV, pp. 301 

 and 308). 



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