THE IMAGINARY RANGE AND TROUGH. 61 



from the southern edge, the negative effect of the trough would be 

 neutralised ; still further south the effect of the compensation would 

 outweigh that of the trough and produce a small positive effect 

 on the force of gravity. 



Calculations were not made for other sections and dimensions 

 of the trough, as an estimate, sufficiently accurate for the purpose 

 of this investigation, may be made for any section of trough which 

 will have to be considered, and, besides, there is the uncertainty 

 of whether the trough should be considered as having a separate 

 compensation of its own. 



One more condition must be considered ; besides the possibility 

 of the separate compensation of the trough there is the possibility 

 that its origin is due to a depression of the crust into, or a subsidence 

 of the crust due to a removal of, the denser material below. In 

 either case there would be a replacement of denser by less dense 

 material of the same shape and form as the trough itself, but 

 situated at some depth below it, as is illustrated in fig. 6. 



Fig. 6. 



An hypothesis of this form has actually been investigated by 

 Mr. 0. Fisher, 1 who supposed the depression of the Gangetic trough 

 to be due to a bending down of the earth's crust in partial support 

 of the weight of the mountain range, but the same result mi»ht be 

 brought about in other ways. On any form of hypothesis, which 

 involves an isostasy and support of the range by flotation, it is con- 

 ceivable that the support might not be completed under the range 

 itself, but partly transferred to the crust on either side, with the 

 consequence of an equal displacement of the denser material under 

 the crust by the lighter material of the crust itself. The bearing 



1 Phil. Mag., Jan. 1904, pp. 14-25. 

 [ 209 ] 



F 



