y 6 OLDHAM : THE STRUCTURE OF THE HIMALAYAS, ETC. 



trough follows the general course of the boundary of the peninsular 

 rock" area, keeping clear of the irregularities and deep indentations 

 of the alluvial boundary, till, west of the 80° meridian, it trends more 

 northwards and from Agra to Delhi runs about north-north-west. 

 In this part of the course the latitude stations help but tittle, as the 

 effect of the trough on the plumb-line would be in a nearly westerly 

 direction, but the gravity observations enable us to place the bound- 

 ary not far west of Hathras, Chandaos, and Gtesupur ; the great 

 spread of alluvium, with rocky hills rising from it, to the westward 

 of this line being merely a covering, of a few hundreds of feet at most, 

 over the rocky floor. 



From Delhi, where the last outlier of the Aravalli Hills disappears 

 below the alluvium, the boundary of the trough must bend nearly 

 west, for we find it running south of Rakhi and close to Ferozepore 

 and Lahore ; the further course cannot be traced with certainty, 

 but the trough appears to be represented as far west as the station 

 oi Ranjitgarh. and probably terminates on the west as the Salt 

 Range is reached, just as it ends up on the east where the Assam 

 Range impinges on the Himalayas. 



From the southern (^ of the trough the floor slopes downwards 

 towards the hills, teaching a depth of probably over 20.000 feet in 

 the broadest part of the trough between 80° and 84°. Near 78° the 

 greatest depth has sunk to not more than 15,000 feet, but further west 

 the maximum depth of alluvium seems to increase again and may 

 rise to .is much as -20,001) I'eet under the plains of the upper Punjab. 



We have, therefore, a fairly symmetrical trough, ranged along 

 the whole of that part of the length of the Himalayas which is not 

 complicated by the junction or contact of other ranges ; and it is to 

 be noted that the symmetry is in reality greater than appears on 

 the map, for the very marked break in the even sweep of the bound- 

 ary, and the prominence culminating near Delhi, are not entirely, 

 and need not be in any way, connected with an irregularity in the 

 displacements by which the trough was produced. The prominence 

 lies on the direct continuation of the Aravalli hills, which still stand 

 out, in the southern portion, as a distinct range of hills, rising above 

 the general level of the country on either side, and the termination 

 of the range to the northwards is not in any way connected with its 

 structure, but solely due to a gradual lowering of the general ele- 

 vation, which has allowed the alluvium to invade the valleys to a 

 greater and greater extent, leaving the higher peaks standing out 



I. 244 j 



