Status of the Theory of Isostasy. 



66 I 



form of a series of blocks which should have the same 

 mass and gravitative effect as the Tibetan plateau, the 

 Himalaya mountains, and the Siwalik foothills. He then 

 constructed tables showing their effect and that of vari- 

 ous forms of compensation upon the deflection of the 

 vertical at various distances. Next he chose several 

 forms of the Gangetic alluvial trough filled with alluvium 

 of density 2-2 and computed the gravitative effects of 

 these, both compensated and uncompensated, upon the 

 vertical at various distances. The effects on the intensity 

 of gravity were also ascertained. The variety of tests 

 formed a series and avoided the arbitrary selection of 

 a special hypothesis^ Using these tables, there were 

 found to be certain limits which would give computed 

 results agreeing closely with those observed. To obtain 

 this agreement it was necessary to introduce the hy- 

 pothesis of regional compensation, implying the exist- 

 ence of a strong crust lying on a denser and weaker 

 substratum. Oldham states this conclusion as follows : 



"We can now summarise the separate conclusions which have 

 been reached, and attain an understanding of the general distri- 

 bution of the compensation of the Himalayas. In the central 

 part of the range the compensation is in excess of the load which 

 it is supposed to support ; in the outer Himalayas, at a distance 

 of 30 to 40 miles in from the edge of the hills, it is in very con- 

 siderable defect ; and somewhere between these two regions must 

 come a tract where the compensation and topography are in 

 adjustment with each other, where, in other words, the anomaly 

 of gravity should be zero, proper allowance being made for the 

 effect of compensation. Towards the outer edge of the hills the 

 defect of compensation diminishes and the anomaly must ulti- 

 mately become zero once more. 



"A variation of this kind in the adjustment between topog- 

 raphy and compensation, or between load and support, is with 

 difficulty intelligible, except on the supposition of a support of 

 the range by flotation, and certainly finds easiest expression in 

 terms of that hypothesis. In the centre of the range the down- 

 ward protuberance of the crust is over-developed and there is an 

 excess of buoyancy, tending to make the range rise ; the excess 

 of load in the outer hills would then be an indication that such 

 rise has taken place, carrying with it the outer hills, till the load 

 thrown on the central tract became large enough to check the 

 further uplift and leave the main range at a lower elevation than 

 that which would result from the protuberance beneath it, while, 

 on the flank of this central tract, the outer hills are upraised 

 beyond the height which they would attain by the effect of the 

 support immediately below them. ' ' 



"It will be seen that this development of the consequences 



