94 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



that the series of longitudinal faults, traversing the Siwalik region, 

 represent successive positions of the boundary between hill and 

 plain, and that the outermost boundary of the hills marks the 

 position of a similar fault, the latest in date of the whole series. 1 



It is now possible to summarise the conclusions drawn from the 

 separate groups of observations and to draw a generalised cross- 

 section of the trough, as is shown in figure 7. This does not represent 

 any one cross-section, for no one cross-section is complete, but, by 

 a combination of the geodetic and geological evidence of different sec- 

 tions, it is possible to represent diagrammatically the general type 

 of section which would be mot with, subject to minor variations, at 

 almost any part of the length of the trough. On the north Ave have 

 the range of the Himalayas proper, and near the southern edge of it 

 a series of faults, which mark the successive boundaries between 

 hill and plain. The outermost and latest of these faults traverse 

 the region where the deposits of the plain have been compressed, 

 folded and elevated into the foot-hills of the Siwalik zone, the outer 

 limit of which is probably marked by a similar fault. 



1 In Vol. VII nf the Records of the Surra/ of India, p. 151, particulars are given of the 

 deflection of the plumb-line at two 'stations between Rajpur and Mussooric. The 

 deflections, in the meridian and the prime vertical, are 



Mussooric :f(i"-f>N 28*2 B. 



.Iharip.mi ',2"-5 N 33*6 E. 



Spin- Point 63*2 N 3l # -3 E. 



Rajput 47"-7N 31*3 E. 



It will be seen that the deflect ions at dharipani and Spur Point are distinctly greater 

 than at Rajpur : pari of this excess is doubtless due to the effect of quite local topography, 

 but these stations are situated rather less than a mile and about half a mile, respectively, 

 from the outcrop of the main boundary fault, that is, in positions where the effect of the 

 trough would be markedly less, and lesser deflections looked for, wcro the plane of the 

 boundary fault vertical. Not so, however, if the fault had a hade towards the hills, as 

 is indicated by the surface geology ; in this case the maximum effect of the trough would 

 be met with to the northwards of the outcrop, and there would not be the same rapid 

 fallinsc-ofT of the deflections as in the case of a vertical plane of separation between the 

 denser and the less dense rocks. Seeing that a hade of 30° from the vertical would bring 

 the fault directly under .lharipani at a depth of 7,000 to 8.000 feet, figures in good accord 

 with the geological and the geodetic observations, the effect of the trough would be at 

 least as great at Jharipani and Spur Point as at Rajpur, the effect of the range would not 

 be materially different and that of the quite local topography perhaps a little greater. 

 The large deflections at the new stations are, therefore, in completo accord with what 

 was to be expected, and confirmatory of the structure which had been deduced from 

 geological examination. 



These observations did not reach me in time to be embodied in the text ; the absolute 

 deflections are liable to modification in the manner which has been indicated, and are 

 actually different from the figures printed above, but this does not affect the differences 

 between the deflections at the different stations. 



L 242 J 



