R. D. Oldham — Origin of the Himalayas. 535 



Siwaliks of the sub-Himalayas and the alluvium of the Gangetic 

 plain. The Siwaliks were long ago shown, by Mr. H. B. Medlicott, 

 to have been formed from the vs^aste of the Himalayan range, under 

 exactly similar conditions, and by the same rivers, as the alluvial 

 deposits of the Gangetic plain, in other words to be merely the lower, 

 older, and marginal, deposits of the same formation, now uplifted and 

 exposed to denudation. The throw of the boundary fault cannot be 

 measured directly, but it is certainly great, and may reasonably be 

 estimated at between 10,000 and 15,000 feet, say between two and 

 three miles. It may exceed or fall short of these limits in places, 

 but is not likely to do so to any matei'ial degree. 



On its southern margin the alluvium thins out over an old land 

 sux'face. Between the two margins nothing can be determined, by 

 direct observation, of the form of the rock floor, but the most natural 

 deduction is, that the thickness of alluvium gradually increases from 

 south to north, reaching its maximum at the great boundary fault, 

 so that the Gangetic trough may be regarded as having the form of 

 a very acute-angled wedge lying on its side, with the thick end 

 towards the north. Mr. H. H. Hayden has recently shown that the 

 pendulum observations of the Indian Survey support this interpre- 

 tation,^ but whether this exactly represents the case or not it is 

 certain that the northern limit of the Gangetic trough is nearly 

 vertical and of a depth of two or three miles, while at the southern 

 limit the thickness of the alluvium is very small. 



The effect on the direction of the plumb-line of a depression of 

 this size and shape, filled with material which cannot have a density 

 of more than 2-2, must be considerable, and I have had the curiosity 

 to investigate it. The detailed results of the investigation would 

 take up too much space to reproduce them here, nor are they suitable 

 to this Magazine, but the general result may be indicated. I find 

 that at the northern limit of the plain tlie effect would be an apparent 

 repulsion of the plumb-bob, or' in other words an apparent excess of 

 attraction by the Himalayas, amounting to about 30" if the depth 

 of the alluvium is taken at 3"5 miles, and 18" if it is taken at 

 1"75 miles. These values are not materialh^ affected by any variation 

 in the width of the alluvium. On either side of the boundary fault 

 the deflexions decrease rapidlj^, but more rapidly to the south than 

 on the north, for whereas, at 20 miles from the boundary on the 

 north, there is still a deflexion of some 5" or 6" to the northwards, 

 the northerly deflexion lias almost disappeared at the same distance to 

 the south, to be replaced, still further south, by a southerly deflexion, 

 or apparent repulsion away from the Himalayas. 



I have only indicated here the general nature of the effect which 

 would be produced, but enough has been said to show that both in 

 kind and magnitude it is very similar to that which has been 

 observed, and for the explanation of which the 20 mile deep rift has 

 •been offered. Kurseong lies about 2 miles north of the main 

 boundary fault, Jalpaiguri lies in the region where southerly 

 deflexions should be expected, and the difference in deflexion, as 



^ Eec. Geol. Surv. Ind., xliii, pt. ii, pp. 163-7. 



