R. D. Oldham — Esmys in Theoretical Geology. 17 



the Tertiary and pre-Tertiary formations. In Mr. Medlicott's memoir 

 this great fault was called the "main boundary " — a name which has 

 been found so convenient that it has frequently since been used in 

 the publications of the Geological Survey of India, and will be 

 retained here in the sense indicated above. 



This main boundary fault is almost everywhere a clean cut 

 fracture, generally reversed, and as a rule the rocks on either side 

 show little or no signs of that crushing which one would expect to 

 have accompanied so great a displacement. As we have seen, it 

 marks, approximately at least, the limit between what was, in 

 Siwalik times, an area of subsidence and an area of elevation, and 

 it is not unreasonable, on this ground alone, to suppose that it was 

 the actual boundary, and formed fari passu with the deposition of 

 the beds to the south. 



But the main boundary is not the only fault of its kind, though 

 the most remarkable ; for, both in the Himalayan and sub-Himalayan 

 areas, there are similar great faults with an upthrow on the 

 Himalayan side, which have been held to mark successive stages 

 in that southerly advance of the margin of the hills, which is 

 sufficiently proved by the elevation of the Siwalik beds, originally 

 deposited in the plains along the foot of the Himalayas. 



This supposition derives considerable support from a consideration 

 of the nature of the boundary between the disturbed Upper Siwaliks 

 and the recent deposits of the plains. With a few exceptions where 

 uppermost Siwalik conglomerates dip towards and under the recent 

 gravels, the boundary is a well-defitied one, and the undisturbed 

 recent gravels are in contact with disturbed sandstones from which 

 several thousand feet of overlying strata have been removed by 

 denudation. There are no great irregularities of the boundary or 

 deep embayments of the recent deposits, such as would result from 

 a subsidence of the hills and an encroachment of the recent deposits, 

 such, in fact, as are seen on the southern margin of the alluvium. 

 The beds of which the hills are formed are precisely similar in 

 character, and were deposited under circumstances identical with 

 those of the present submontane deposits ; and this, taken in con- 

 junction with the outline of the boundary, leads one naturally to the 

 conclusion that the latter is a structural one, and that the hills 

 have slowly and contemporaneously been undergoing elevation and 

 denudation. At the same time the greater bulk of the debris 

 washed down from them has been deposited near their foot, and, to 

 make room for this vast quantity of debris, the latter area must 

 have been slowly subsiding. We have consequently, in close 

 contiguity with each other, an area which has been subject to 

 elevation, and one which has been subject to depression, the 

 boundary between the two being necessarily analogous to those 

 faults with an upthrow to the north which intersect the sub- 

 Himalayan region, and to the great boundary fault which limits it 

 on the north. ^ 



^ This question is more fully treated by my colleague, Mr. Middlemiss, in his 

 recently published memoir, Mem. Geol. Surv. Ind. vol. xxiy. pt. ii. 



DECADE III. VOL. VIII. NO. I. 2 



