4 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



(1) Firstly, there is the indubitable fact that the elevation 

 of the Himalayas has been accompanied by the compression of the 

 rocks of which it is composed. It is not meant that the whole 

 of the disturbance of the Himalayan rocks has been the consequence, 

 or the cause, of the elevation of the Himalayas ; the contrary is 

 indeed almost certain, 1 but the general distribution of the rocks, 

 in the larger anticlines and synclines, along the general course of 

 the range, "and the fact that the prevailing strike is in the same 

 direction, point to a connexion between the disturbance of the 

 rocks and the elevation of the range. In the Siwalik region of the 

 foot-hills the connexion is incontestable, for here we find that the 

 rocks, which must have been deposited in practically horizontal beds 

 at a time when the elevation of the Himalayas was already in 

 progress, are now folded, disturbed, and compressed in a direction 

 transverse to the general course of the range. 



(2) There is, along the outer edge of the Himalayas, a great 

 fault, known as the main boundary fault, which separates the 

 northern area of the rocks of the Himalayas from the southern 

 area occupied by the Upper Tertiary Siwalik rocks of the Sub- 

 Himalayas. This fault, as was originally shown by Mr. Medlicott, 

 marks very closely the original limit of formation of the Siwaliks, 

 and the boundary separating an area of elevation and denudation, 

 to the north, from an area of subsidence and deposition, to the 

 south. He also showed that the Siwaliks were formed under the 

 same conditions as the marginal deposits of the Gangetic alluvium, 

 that the material of which they were composed was derived from 

 the Himalayan area, — in other words, that the Himalayan range 

 had already been marked out as an area of special uplift in early 

 pliocene times. Along the greater part of the length of the Hima- 

 layas this fault brings the indurated older rocks of the Himalayas 

 into direct contact with the soft sandstones and shales of the Upper 

 Tertiary series, and throughout this region we have two groups 

 of rocks of vezy markedly different densities separated by a nearly 

 vertical plane of separation. A condition like this cannot but 

 have a marked influence on the direction and amount of the force 

 of gravity and, as will be seen in the sequel, a study of this effect 

 enables us to form an approximate estimate of the vertical depth 

 to which the contrast extends. Between the Jumna and the 



1 Sec Manual of the Geology of India, 2nd cd., p. 483 ; also Records, Geol. Svrv., India 

 XLlll, p. 140. 



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