THE UNDERGROUND FORM OF FLOOR OF GANGETIC TROUGH. 93 



Here, as before, the irregularity shown by the first four stations is 

 probably connected with the distribution of the compensation of 

 the range, and will be dealt with in the next chapter. At the other 

 stations the anomalies show the same feature as the Bouguer values, 

 and indicate an increase in the negative anomaly of about -03 dvne 

 as between stations in the Siwaliks and the nearest .ones on the 

 alluvial plain. Interpreted as an effect of the alluvium, this means 

 an increased depth of about 4,500 feet. 



The general result, then, of an examination of the geodetic obser- 

 vations in the Dehra Dun is that the observations of the deflection 

 of the plumb-line require that the magnitude of the main boundary 

 fault shall be of the order of near 10,000 feet vertical throw ; 

 they suggest the possibility, though they cannot establish the exis- 

 tence, of a rise in the floor of the trough coincident with the outer 

 limit of the Siwalik Hills ; they show that if such a step exists it 

 must mean a rise of some thousands, probably near 5,000 feet ; 

 that in this case the throw of the main boundary fault will be near 

 the lower limit indicated, but will be near the upper limit if the 

 floor of the trough continues under the Siwalik area with no material 

 change in level. Finally, they exclude the possibility of a deepening 

 of the trough under the Siwalik area as compared with its depth 

 under the plains to the south. 



The gravity observations, on the other hand, do not enable us 

 to determine the depth of the trough at the main boundary ; though 

 they indicate that the main boundary fault has a throw of several 

 thousand feet, they do not enable us to decide, directly, between the 

 two alternatives presented by the observations of the deflection of 

 the plumb-line. Indirectly, however, they do give an answer, for 

 they indicate most unmistakeably that there is a very considerable 

 drop in the level of the floor of the trough at, or near, the southern 

 edge of the Siwalik Hills, amounting to something like 5,000 feet in 

 vertical difference, with a depth of somewhere about 10,000 faet on 

 the one side and about 15,000 feet on the other, of the step. 



Taken together, these observations indicate that the boundary 

 of the outer hills, if we could obtain a deep section, would be of very 

 much the same character as the main boundary fault, thus con- 

 firming the suggestion, first made by Mr. H. B. Medlicott ' and sub- 

 sequently worked out in much greater detail by Mr. C. S. Middlemiss, 2 



1 Mem. Geol. Surv. Ind., Vol. III. pt. 2 (1864). 

 * Mem. Geol. Surv. Ind., Vol. XXIV, pt. 2 (1890). 



[ 241 J 



