98 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



To the eastwards there seems to be a much smaller interruption 

 of the regular sweep of the boundary of the trough in the north- 

 eastern extremity of the peninsular area, to the south of Monghyr, 

 which may be due to an original greater elevation of the land surface 

 as compared with the regions on either side. 



Excluding these departures from symmetry, for which quite 

 obvious and adequate causes are apparent, the trough forms a 

 remarkably symmetrical structure extending along the southern 

 face of the Himalayas, from the Salt Range on the west to the 

 Assam Range on the east. A structural feature exhibiting a sym- 

 metry and dimensions so closely coincident with those of the Hima- 

 layan range can hardly be wholly independent in its origin, and any 

 attempt to account for the formation of one must take cognisance 

 of the origin of the other. This is a matter which will be dealt with 

 further on, but it must be pointed out that the trough, whose form 

 and dimensions have been investigated, is something apart from the 

 great spread of alluvium, stretching from the delta of the Ganges 

 to that of the Indus. To this spread of alluvium the term Indo- 

 Gangetic may be applied with perfect propriety, but it would 

 evidently be incorrect to apply that name to the trough seeing that 

 in no part of its course does the river Indus touch or even approach 

 the deep alluvial trough along the foot of the Himalayas. 



There is some reason to suppose that a. deep trough filled with 

 alluvium, similar to that which has been dealt with, though smaller 

 in size, runs along the foot of the hill ranges of the western frontiers 

 of India proper, which might be called the Indus trough, as that 

 river traverses it from end to end. The other may be appro- 

 priately described as the Gangetic trough, seeing that three-quarters 

 of its length and more than that proportion of its area lie within 

 the drainage of the Ganges, but there is no reason to suppose that 

 the two troughs are connected. Apart from the observations which 

 have been dealt with, the outcrops of old rocks in the Chiniot, and 

 other, hills which rise from the alluvium, point to the presence of a 

 rock barrier, stretching under the plains of the Punjab to the Salt 

 Range and separating the two deep troughs. 



I 246 ] 



