120 OLDHAM: iHB Stfe DCTURE OK T1IK HIMALAYAS, ETC. 



one section, that of the Dehra Dun, do the observations extend 

 across the Siwalik area to the Himalayas proper, and here they 

 indicate that the maximum depth of the alluvium lies not far from 

 the outer edge of the Siwalik hills, but whether actually at the 

 boundary or at some distance from it is not established. Under 

 the Siwalik area there is a distinct shallowing of the trough, pro- 

 bably abrupt and coincident with the outer edge of the hills, and, 

 at the northern boundary of the Siwalik region, the floor of the 

 trough rises abruptly along the main boundary fault, the throw 

 of which is indicated as something less than 10,000 but probably 

 over 7,000 feet. 



Another section, which traverses the whole width of the allu- 

 vium near the 81° meridian, but stops short at the foot of the hills, 

 indicates much the same conclusions, that the floor of the trough 

 rises rapidly under the Siwalik area, though here the maximum 

 depth may be 50 miles or more from the edge of the hills. A 

 third section, near the eastern end of the trough, where the Siwalik 

 zone is unrepresented, or covered by alluvium, indicates an increase 

 in depth from south to north almost up to the outer edge of the hills, 

 though a larger number of observations might put the maximum 

 depth somewhat south of the station nearest the hills, at which 

 the largest depth is indicated. 



The structure indicated on these sections may reasonably be 

 extended to others, and in it we find a confirmation of the deduc- 

 tion, which had been drawn from geological data, that the under- 

 ground form of the trough near its northern limit, as well as the 

 nature of the northern boundary, is radically different from what 

 is to be found under the southern part of the trough. To the south 

 of the present line of maximum depth the trough has been formed 

 by simple subsidence and the alluvium deposited on an old land 

 surface, preserved with little or no change in its original form. 

 To the north, the rise is not only more rapid, but more irregular 

 and determined mainly by tectonic processes, connected with the 

 origin of the hills, which have profoundly altered the original form 

 of the floor of deposition, and involved some of the originally un- 

 disturbed deposits in the folding and faulting of the process of 

 mountain formation. 



Incidentally we find -a confirmation of the interpretation which 

 had been accepted, rather than demonstrated, that there is a rise 

 in the floor of the trough under the Siwalik area, and indirectly 



L 208 ] 



