INTRODUCTORY. 5 



Sutlej rivers, older Tertiaries appear on the northern side of the 

 main boundary fault and, beyond the Sutlej, the whole of the 

 Tertiary system becomes involved in the mountain-forming dis- 

 turbances. In this region the main boundary fault is no longer 

 recognisable, having merged into one of a series of more or less 

 parallel faults, of similar character, which traverse the area of 

 Tertiary rocks in the outer Himalaya. 



(3) Mr. Medlicott also showed that the main boundary fault 

 was not the only feature of its kind, for a series of similar faults 

 is found within the Siwalik area, which were regarded as marking 

 successive limits between an area of uplift and erosion to the north" 

 and of deposition to the south of the fault line. This conclusion 

 was more fully worked out by Mr. C. S. Middlemiss J in the Sub- 

 Himalayas of Kumaon and Garhwal, where he showed that not 

 only was there a succession of faults within the Siwalik area, each 

 of later date than the next one to the north and each in succession 

 marking the limit of the region of Himalayan uplift, but that there 

 was also a series of similar faults to the northwards, each in suc- 

 cession earlier than the one to the south and, presumably, marking 

 the successive limits of the Himalayan area ; and a similar con^ 

 elusion is suggested by the geological structure of the Sikkim 

 district. 2 From this it follows that, at any rate during the latter 

 part of the period of elevation of the Himalayas, there has always 

 been an abrupt limit of the region of compression and elevation, 

 and that this boundary has progressively shifted southwards, 3 

 encroaching on an area of deposition and involving deposits of later 

 date in the mountain-forming processes. 



(4) The clearly defined character of the southern margin of 

 the hills towards the plains, running with a regular sweep alom' 

 the foot of the hills, and the absence of detached outliers rising 

 out of the alluvium, irresistibly suggests that the boundary is deter- 

 mined by a structural feature similar to the main boundary and 

 the faults in the Siwalik area, and though no direct measurement 

 of the depth of the undisturbed alluvium is possible, the fact that 

 it is identical with, and a continuation of, the Siwalik deposits 



1 Memoirs, Vol. XXIV, pt. 2. 



2 Memoirs, Vol. XI, pt. 1. 



3 This statement necessarily refers only to the position of the successive boundaries 

 relative to each other. There is no means of deciding whether there has, or has not 

 been any general movement of the Himalayas northwards or southwards, whether 

 in latitude or as regards distunce from the rocks of the peninsular area. 



E 153 ] 



