8 dLDHAM: THE STKLCTUliE UK THE HIMALAYAS, F.TC. 



a couple of thousand feet, and on most sections even less, so that, 

 even allowing for the extensive removal of material, and lowering 

 of the hills, by denudation, there is a possibility that the lloor of 

 the Siwaliks is not materially higher, and may even be lower, than 

 that of the alluvial deposits immediately beyond them. 



(4) As has been stated, we have very good reason for supposing 

 that the thickness of the alluvial deposits, along the southern edge, 

 of the hills, is not less than some 15,000 feet; we also know that 

 the thickness near the southern edge is very small, but we have 

 no direct knowledge of what takes place between these limits, 

 whether the depth of alluvium is at its maximum near the northern 

 edoe and gradually diminishes to the southwards, or whether it 

 inereases to a maximum and then diminishes, or whether it con- 

 tinues with a considerable depth to near the southern edge ami 

 then thins out rapidly. In other words, we are unable to draw 

 a cross section of the Gangetic trough J with any degree of 

 certainty. 



(5) Though the alluvial areas of the Gangetic and Indus drainage 

 areas are continuous with each other, and the whole area is coloured 

 uniformly on the geological map, it has been recognised that there 

 is a considerable difference in the surface contour, in the arrange- 

 ment of the river courses, and in the character of the deposits which 

 form the surface of the two regions. From the Jumna eastwards 

 to the junction with the Brahmaputra Valley is the great tract 

 of the typical Gangetic alluvium, which bears all the characters 

 of a plain of deposit and across which the rivers flow in courses 

 determined by their own action and interaction. In the plains 

 of the Punjab these features are largely absent, and the surface 

 features suggest a much smaller thickness of alluvial deposit, a 

 suggestion which is strengthened by the occurrence of inliers of 

 older rocks, rising as hills in the centre of the alluvial plain. 



1 The titlo of a paper by Sir S. Q. liurrard, published in Pror. Hoy. She., Series A , 

 XCI, p. 221, 'On the origin of the Indo-Gangetic Trough, eonunonly called the Hima- 

 layan Foredeep,' is liable to convey a wrong impression. The basin filled by the tndo- 

 Gangetic alluvium is certainly not commonly called the Himalayan Foredeep. and the 

 use of the terms as synonymous is improper. The word " foredeep " occurs in Prof. 

 Sodas* translation of Dm Antlitz dcr Erdc as the English equivalent of the word 

 Vortiejc. coined by Prof. Sucss with the intention of conveying not only a description, 

 but also a definite theory of origin. The word may be used without accepting this 

 theory, but a term, whio'h was invented to connote a definite theory of origin, cannot 

 be used with propriety unless that theory is intended to be implied. I shall confine 

 myself to the use of the word trough, which is purely descriptive and implies no theory 

 of origin, and in using it shall refer only to the deep depression in the rock surface 

 under "the alluvial plain, not to the whole of tho area which is mapped as ulluvmm. 



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