THF UNDERGROUND FORM OF FLOOR OF GANUETIC TROFGR. 97 



as rocky inlicrs in the alluvium, till the range finally disappears 

 in the last exposure of rock at Delhi. There is, however, nothing to 

 suggest that the range docs not continue northwards under the 

 alluvium, with the same irregular surface and general elevation 

 above the rock surface on either side, and the geodetic observations 

 indicate the same conclusion. The northerly deflections at Datairi 

 and Boston occur on the direct continuation of the line of the main 

 range of the Aravallis, and beyond them the gravity observation 

 at Meerut indicates a smaller negative anomaly, which may be 

 interpreted as a lesser depth of alluvium than is found in other 

 similarly situated stations. Still further in the direction of the 

 Himalayas the comparatively small northerly deflection at Sarkara 

 suggests a lesser depth of alluvium under this station than under 

 similarly situated stations further east ; and if the line is continued into 

 the Himalayas it strikes a region where the geological structure has 

 suggested the possibility of an original extension of the Aravalli 

 range into what is now the Himalayan region 1 ; the geodetic observa- 

 tions have supported this suggestion and converted what was only 

 a bare possibility into something more than a probability. 



The existence of a structural feature of such magnitude as the 

 Aravalli Kangc, extending across the region where the Gan<*etic 

 trough was subsequently brought into being, would have a twofold 

 effect. In the first place it would introduce a variation in the 

 strength of the earth's crust, and so a difference in the resistance 

 which it would offer to the forces by which the trough was produced, 

 and in the second place the mere fact of the original greater surface 

 elevation of the range would result in the country on either side 

 being soonest brought below the level of the formation of alluvium, 

 and so give rise to an indentation in the boundary and a projection 

 of rock into the alluvial area, quite apart from any possible difference 

 in the amount of the surface warping, by which the trough was 

 produced. In this way the northerly deflections at Datairi and 

 Bostan, which it must be remembered are only northerly if the 

 deflection at Kalianpur is assumed to be as much as 4" to the south, 

 represent the absence of a regular shallowing of the alluvium to the 

 southwards, or the presence of very considerable irregularities of 

 the under surface, no less than a deficiency of depth, so that the 

 influence of uneven distribution of density in the underlying crust 

 ceases to be masked by the effect of the trough. 



1 Manual, 2nd cd, p. 483. 



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