460 Notices of Memoirs — Australia, 1914 — 



may be such as to give rise to different effects on the pendulum. If, for instance, 

 one considers two columns of the same size and of exactly the same weight, with, 

 in one case, the heavy material at a high level and in the other case with the 

 heavy material at a low level, the centre of gravity of the former column, being 

 nearer the surface, will manifest itself with a greater pull on the pendulum ; 

 these columns would be, however, in isostatie adjustment.^ 



Gilbert's hypothesis thus differs slightly from the conception put forth by 

 Hayford and Bowie ; for Gilbert assumes that there is still appreciable hetero- 

 geneity in the more deep-seated parts of the Earth, while Hayford and Bowie's 

 hypothesis assumes that in the nuclear mass density anomalies have practically 

 disappeared, and that there is below the depth of compensation an adjustment 

 such as would exist in a mass composed of homogeneous concentric shells. 



In order to make the Indian observations comparable to those of the United 

 States as a test of the theory of isostasy, Major H. L. Crosthwait^ has adopted 

 Hayford 's system of computation and has applied it to 102 latitude stations and 

 eighteen longitude stations in India. He finds that the unexplained residuals 

 in India are far more pronounced than they are in the United States, or, in 

 other words, it would appear that isostatie conditions are much more nearly 

 realized in America than in India. 



The number of observations considered in India is still too small for the 

 formation of a detailed map of anomalies, but the country can be divided into 

 broad areas which show that the mean anomalies are comparable to those of 

 the United States only over the Indian Peninsula, which, being a mass of rock 

 practically undisturbed since early geological times, may be regarded safely as 

 having approached isostatie equilibrium. To the north of the peninsula three 

 districts form a wide band stretching west-north-westwards from Calcutta, with 

 mean residual anomalies of a positive kind, while to the north of this band lies 

 the Himalayan belt, in which there is always a large negative residual. 



Colonel Burrard^ has considered the Himalayan and sub-Himalayan 

 anomalies in a special memoir, and comes to the conclusion that the gravity 

 deficiency is altogether too great to be due to a simple geosynclinal depression 

 filled with light alluvium such as we generally regard the Gangetic trough to 

 be. He suggests that the rapid change in gravity values near the southern 

 margin of the Himalayan mass can be explained only on the assumption of the 

 existence of a deep and narrow rift in the sub-crust parallel to the general 

 Himalayan axis of folding. A single large rift of the kind and size that Colonel 

 Burrard postulates is a feature for which we have no exact parallel ; but one 

 must be careful not to be misled by the use of a term which, while conveying 

 a definite mental impression to a mathematician, appears to be incongruous 

 with our geological experience. There may be no such thing as a single large 

 rift filled with light alluvial material, but it is possible that there may still be 

 a series of deep-seated fissures that might afterwards become filled with 

 mineral matter. 



With this conception of a rift or a series of rifts, Colonel Burrard is led to 

 reverse the ordinary mechanical conception of Himalayan folding. Instead now 

 of looking upon the folds as due to an overthrust from the north, he regards 



^ It is interesting to note that the idea suggested by G. K. Gilbert in 1913 

 was partly anticipated by Major H. L. Crosthwait in 1912 (Survey of India, 

 Professional Paper No. 13, p. 5). Major Crosthwait, in discussing the similar 

 gravity anomalies in India, remarks parenthetically : "Assuming the doctrine of 

 isostasy to hold, is it not possible that in any two columns of matter extending 

 from the surface down to the depth of compensation there may be the same 

 mass, and yet that the density may be very differently distributed in the two 

 columns ? These two columns, though in isostatie equilibrium, would act 

 differently on the plumb-line owing to the unequal distribution of mass. The 

 drawback to treating this subject by hard and fast mathematical formulae is 

 that we are introducing into a discussion of the constitution of the earth's crust 

 a uniform method when, in reality, probably no uniformity exists." 



- Survey of India, Professional Paper No. 13, 1912. 



'^ Ibid., No. 12, 1912. 



