88 MIDDLEM1SS: PHYSICAL GliOLOGY OF SUB-HIMALAYA. 



sandstone suddenly hardens again and then ascends gradually to the 



main boundary. 



There are three possible explanations of the dying out of the 



, , , . sand-rock bands in this direction. We may 

 Nature of the dying 



out of the M. Siwalik suppose (i) that they were deposited conti- 



stage in this direction. 



nuously over the area we are now entering 

 upon, which was then upheaved, and they were denuded away before 

 the two thrust planes came into existence ; (2) that they were 

 deposited continuously and then thrust over and buried by the Nahans, 

 each in the form of a detached trough core (noyau synclinal dStache 

 par etranglement) 1 ; (3) that they were not deposited over this part 

 of the country at all, or, in other words, that the Nahans here were 

 elevated before the M. Siwalik age. I think a combination of the 

 first and third of these suppositions the correct interpretation. It 

 would be difficult to account for their sudden cessation west of the 

 Kotri and Chokamb duns entirely by thinning out, because there is 

 such a very great thickness of them seen along the section up the 

 Kotri stream. On the other hand, it would be also difficult to account 

 for them entirely by the first or second hypothesis, because the 

 reversed faults manifestly cease a short distance west of the Kho, 

 there being no trace whatever of them in the Rausan N. in that 

 direction. We may conclude then, that partly by thinning out, due to 

 elevation, and partly by reversed faulting, the two bands of M. 

 Siwalik sand-rock have thus come to a rather sudden termination. 

 The section up this stream shews that, though the Nahan zone is 

 as wide here as in the Kho, there has been no 

 piling up of the strata, no reduplication of them 

 by reversed faulting. The angle of dip is consequently low in a gra- 

 dually ascending series, from harder Nahan beds with purple shales 

 at the south end of the stream, to softer Nahans at the main boundary. 

 The latter are not sufficiently high in the stage to be called sand- 

 rock. 



1 See " Les Dislocations de Pecorce terrestre " par Emm. de Margerie and Dr. Albert 

 Heirrr, p. 60. 



( >46 ) 



