116 OLDHAM: THE STRtJCTUEB OF T11K HIMALAYAS, ETC. 



compensation to the topography, not necessarily coincident with 

 the zero points of the Hay lord anomaly ; to the left there is an ex- 

 cess of compensation, resulting in a tendency of the range to rise 

 as indicated by the arrows ; between the two zero points the com- 

 pensation is in defect and the excess of load results in a tendency 

 for the hills to sink. The same conclusions may be otherwise 

 depicted in the outline of the topography where the firm line re- 

 presents the section of the range as it actually exists, and the dotted 

 line that which it should be if the topography were everywhere 

 adjusted to the compensation. 



One more conclusion may be drawn from the distribution of 

 stresses indicated in the diagram. If the crust has suilicicnt 

 strength to bear the load imposed on it by the superelevation of 

 the outer hills, it is improbable that the adjustment would cease 

 at the right hand zero point ; the load on the tract between the 

 two zero points would tend not only to hold down the central por- 

 tion of the range trom rising but also to bear down the crust on 

 the right into the plastic, denser, layer below, and so we might 

 expect to find a defect of gravity outside the range, quite apart 

 from that due to the defect in density of the alluvium. It will 

 be shown, further on, that there is some indirect evidence of the 

 existence of such a depression of the under side of the crust, but 

 there is no possibility of getting any direct confirmation of it 

 from observations in the alluvial area, for the effect would be 

 masked by that of the trough ; and as our only estimate of the 

 depth of the trough, except close to its margins, is derived from 

 the geodetic evidence, any attempt, based on this evidence, to 

 separate out the one effect from the other would be merely arguing 

 in a circle. 



The local departures from a condition of equilibrium between 

 topography and compensation, which have been found in the Hima- 

 layas, indicate a degree of rigidity, and strength, of the crust greater 

 than that which has sometimes been attributed to it, and this might 

 lead to doubt as to the correctness of the inferences which have been 

 drawn. On this point we have, fortunately, the recent elaborate 

 investigation of the rigidity of the earth's crust by Prof. J. 

 Barrell, 1 in which, after dealing with geological and geodetic data 

 in the United States and elsewhere, he concludes that the crust 

 is strong enough to support a load of over 3,000 feet of rock, har- 



1 Journal of Geology, Vols. XXII and XXIII pturim, 1914-15. 



L 264 ] 



