14 R. JD. Oldham — Essays in Theoretical Geology. 



has commenced, though not so fullj'^ developed as further to the 

 south-east. Next to the pre-Tertiary rocks of the Himalayas proper 

 comes a bi'oad zone, principally formed of the Murree group, 

 at the outer margin of which rise some inliers of pre-Tertiary 

 (Carboniferous ?) limestone, overlaid by the marine Subathu group, 

 while near the inner boundarj'^, with the pre-Tertiary rocks, there 

 are some outliers of Upper Siwalik clays and conglomerates. The 

 general section from S.W. to N.E. is therefore an ascending one, 

 and both boundaries of this zone are, in effect, great faults with 

 an upthrow on the Himalayan side. Between this zone of Murree 

 beds and the alluvium of the plains there comes a band of Upper 

 Tertiaries, which, on the only published maps, are divided into an 

 inner, older, and an outer, newer zone ; but there is no indication of 

 the nature of the boundary. 



After leaving the Jamu Hills, the zone of Lower Tertiaries 

 becomes much restricted in width, till we reach the Simla region. 

 Here the Lower Tertiaries, forming the Sirmur series, are distinctly 

 separated from the Upper Tertiaries or Nahan and Siwalik groups. 

 The boundary is a great reversed fault, and along nearly the whole 

 length of it the two divisions are not in contact, but separated by 

 a band of pre-Tertiary slates and limestones. The beds of the 

 Sirmur series have partaken of the intense disturbance of the 

 pre-Tertiary beds, with which they are folded up in the most com- 

 plicated manner, and the northern boundary is no longer a single 

 well-defined fault, to the north of which there are no outliers. But 

 the most important point to notice is that in the area occupied by 

 the Sirmur series there is no trace of the Upper Tertiary groups, 

 which were either never deposited or have been completely removed 

 by denudation. 



Still further to the south-east, the small outliers in Garhwal only 

 exhibit the remains of the Subathu group, no representative of the 

 two Upper groups of the Sirmur series being found. 



The explanation of these facts, which has been adopted by every 

 observer of the ground in question, is that the disturbance of the 

 Himalayas gradually advanced from S.E. to N.W., and that the 

 missing upper groups were never deposited over the outliers owing to 

 the elevation resulting from this disturbance. There is ample 

 corroborative evidence to prove the truth of this hypothesis ; but 

 the same result is arrived at even if we suppose that the full 

 sequence of deposits was formed everywhere alike, for the condition 

 of the Simla area is such as would result from a greater elevation 

 and denudation of the Jamu area, while the outliers in Garhwal 

 might result from a still further extension of the same processes. 

 But as both elevation and denudation take time, and as the con- 

 ditions of the three areas with regard to denudation at least are 

 substantially similar, we reach the same conclusion as before, that 

 the elevation, and consequently the denudation of the Garhwal area 

 must have commenced before that of the Simla area, and this again 

 before that of the Jamu Hills. 



Before passing on to a consideration of the evidence of the 



