16 R. D. Oldham — Essays in Theoretical Geology. 



west of the Jumna : but further west, in the country south of Nahan, 

 the conglomerates not only decrease in thickness, but change their 

 character. The well-rounded boulders of crystalline rocks cease, 

 and, in their place, we find imperfectly rounded fragments of lime- 

 stone, slate, and quartzite derived from the Lower Himalayas, mixed 

 with a large proportion of fragments of Lower Tertiary sandstones. 



Where the Sutlej passes out through the Siwalik area, there is 

 again a large development of Upper Siwalik conglomerates and the 

 same features of a prevalence of conglomerates near the great rivers, 

 and their absence away from them, continues all the way to the 

 Jhelum.^ 



This peculiarity in the distribution of the Siwalik conglomerates 

 shows that not only are the Siwalik beds of subaerial origin ; but 

 that, when they were formed, the Himalayas existed as an elevated 

 region whose main features of hydrography had already been marked 

 out, and further the occurrence of coarse conglomerates, which could 

 only have been deposited in the immediate neighbourhood of the 

 limit of hill and plain, shows that the southern margin of the 

 former must at that time have been approximately the same as the 

 present boundary between the rocks of the Himalayas and of 

 the Siwalik series. 



Taking all these considerations into account, the conclusion becomes 

 inevitable that the Siwalik beds were deposited subaerially and under 

 conditions similar to those of the recent deposits along the foot of 

 the Himalayas ; that the latter existed as a mountain range, com- 

 parable to that now existing, in which the main features of its 

 hydrography had already been marked out ; and consequently that 

 the original northerly extension of the Siwaliks cannot have very 

 far overstepped their present northern boundary. 



The thickness of these subaerially formed beds varies considerably, 

 but is always great ; from ten to twenty thousand feet and perhaps 

 more in places ; and as they now, after having been compressed, 

 disturbed and elevated, do not occur at a height of more than a few 

 thousand feet above the sea, it is evident that they must have been 

 deposited in an area of continuous subsidence, which approximately 

 kept pace with the deposition. On the other hand, the Himalayan 

 area, as I shall subsequently show, has been in a state of continuous 

 elevation throughout the Tertiary period, and this elevation has more 

 than kept pace with the denudation of the surface. We have con- 

 sequently two parallel regions, one of elevation accomj^anied by 

 denudation ; the other of dei^osition accompanied by subsidence, 

 and we will now investigate the nature of the boundary between 

 them. 



Everywhere along the Himalayas the present boundary, between 

 the Upper Tertiary beds and those of pre-Tertiary age, is a gigantic 

 reversed fault. It is one of the most conspicuous structural features 

 of the Himalayas, and, except for the outliers of Lower Tertiary 

 beds in the Simla district and in Garhwal, and the inliers of pre- 

 Tertiary rooks in Jamu, forms an absolute line of demarcation between 

 1 Medlicott, Eec. Geol. Siirv. India, vol. ix. p. 57. 



