332 Status of the Theory of Isostasy. 



Isostasy in India. 



India is the birthplace of the hypothesis of isostasy, 

 through Archdeacon Pratt showing in 1855 that the 

 Himalaya mountain system deflected the vertical at sta- 

 tions near the mountains much less than should have 

 been the case in view of the mass of matter above sea 

 level. It was pointed out by Airy that the simplest 

 explanation lay in the assumption that the crust below 

 the mountain system was less dense than that beneath 

 the plateau of India. 



The comprehensive scope of the geodetic and topo- 

 graphic survey of India enabled Crosthwait in 1912 to 

 apply the Hayford method of computation to the data 

 on the deflections of the vertical. 31 His investigation 

 showed that, under the Hayford hypothesis of uniform 

 compensation to depth of 113-7 kilometers, the mean dif- 

 ference between calculated and observed deflections for 

 India is 5-1". The regions of very high residuals are 

 in Region No. 1, in the foothills of the Himalayas, where 

 the mean residual is — 16", and Eegion No. 3, northeast 

 India, consisting of the broad alluvial basin of the Ganges 

 and bordering upland to the south, where the mean resid- 

 ual is +8". 



Crosthwait concludes that isostatic conditions are much 

 more nearly realized in America than in India, as judged 

 by the size of the residuals, and points out the probable 

 cause in the great mountain-building forces which in 

 recent geologic times have given rise in the north of 

 India to the culminating range of the whole surface of 

 the globe. 



These discrepancies with the particular hypothesis of 

 isostasy adopted by Hayford have received various in- 

 terpretations. Col. S. Gr. Burrard has made them the 

 basis of a geological interpretation 32 which contains such 

 apparent improbabilities as to be impossible of accept- 

 ance without better proof than has yet been submitted. 



H. H. Hayden has tested the data by assuming other 

 depths of compensation, and has found that among 

 several solutions the mean of the computed deflections 

 approaches closest to the observed deflections if the 

 compensation beneath the Himalaya extends to a depth 



31 H. L. Crosthwait, Investigation of the theory of isostasy in India, Prof. 

 Paper No. 13, Trigonometrical Survey of India, 1912. 



3 * S. G. Burrard, On the origin of the Himalaya mountains, Prof. Paper 

 No. 1~, Trigonometrical Survey of India, 1912. 



