26 



OLDHAM: THE STKUCTUKE OK THE HIMALAYAS, ETC. 



only true of a simple layer of rock 100 feet in thickness, but are 

 equally true of a proportionate excess or defect of density, distri- 

 buted through a greater thickness in such a manner that the mass 

 of any vertical column of rock, which wholly includes the cone, is 

 in excess or defect by the equivalent of 100 feet of rock of average 

 density. 



This formula, and the figures derived from it, will be useful 

 in comparing the effect which should be expected on different 

 hypotheses of the nature and distribution of compensation. The 

 way in which these differences arise will be most readily explained 

 by a reference to fig. 3, which represents the case or a station S 

 on the surface of an elevated plateau ; it will be affected by the 



Fig. 3. 



downward pull of the mass of rock lying between the level of S 

 and sea level, represented by I) 1), it will also be affected by 

 a defect of downward pull, due to the diminution of density by 

 which the weight of the plateau above D JJ is compensated. 

 To simplify the consideration of the relative effects of these two 

 forces we. will suppose that the variations of density are so dis- 

 tributed that the centre of effect of any column of small area lies 

 at the depth A A in the case of the plateau, and of C C in the 

 case of the compensation ; and that the line S M represents that 

 along which any given mass will produce a greater effect at S than 

 if it were situated at a greater or less depth on the same vertical 

 line. 



Now take the case of a small column so situated that the centre 

 of attraction of the plateau at A' is on the line S M, and the centre 

 of compensation vertically below it at C ; here the effect of the 



I IT* ] 



