354 TRANSACTIONS OF SECTION C. 
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 
18 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 isostatic conditions are much more nearly realised 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 isostatic 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 
the corrugations to be the result of an under-creep of the sub-crust towards the 
north. Thus, according to this view, the Himalaya, instead of being pushed 
over like a gigantic rock-wave breaking on to the Indian Horst, is in reality 
being dragged away from the old peninsula, the depression between being filled 
up gradually by the Gangetic alluvium. So far as the purely stratigraphical 
features are concerned, the effect would be approximately the same whether 
there is a superficial overthrust of the covering strata or whether there is a 
deep-seated withdrawal of the basement which is well below the level of 
observation. 
Since the Tibetan expedition of ten years ago we have been in possession 
of definite facts which show that to the north of the central crystalline axis 
of the Himalaya there lies a great basin of marine sediments forming a fairly 
complete record from Paleozoic to Tertiary times, representing the sediments 
columns? These two columns,- though in isostatic 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 
formule 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.’ 
23 Survey of India, Professional Paper No, 13, 1912. 
24 Ibid. No. 12, 1912. 
— ed 
