Address by Prof. Sir T. H. Holland. 461 



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, tlie 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 Palfeozoic to Tertiary times, representing the sediments 

 which were laid down in the great central Eurasian ocean to which Suess gave 

 the name Tethys. We have thus so far been regarding the central crystalline 

 axis of the Himalaya as approximately coincident with the old northern coast- 

 line of Gondwanaland ; but, if Colonel Burrard's ideas be correct, the coastline 

 must have been very much further to the south before the Himalayan folding 

 began. 



Eepresenting what the Geological Survey of India regards as the orthodox 

 view, Mr. H. H. Hayden ' has drawn attention to some conclusions which, from 

 our present geological knowledge, appear to be strange and improbable in 

 Colonel Burrard's conclusions, and he also offers alternative explanations for 

 the admitted geodetic facts. Mr. Hayden suggests, for instance, that the 

 depth of isostatic compensation may be quite diii'erent under the Himalayan 

 belt from that under the regions to the south. His assumptions, however, in 

 this respect are, as pointed out by Colonel G. P. Lenox Conyngham," at variance 

 with the whole theory of isostasy. Mr. Hayden then suggests that most of the 

 excessive anomalies would disappear if we took into account the low specific 

 gravity of the sub-Himalayan sands and gravels of Upper Tertiary age as 

 well as of the Pleistocene and recent accumulations of similar material filling 

 the Indo-Gangetic depression. It would not be at all inconsistent with our 

 ideas derived from geology to regard the Gangetic trough as some 3 or 4 miles 

 deep near its northern margin, thinning out gradually towards the undis- 

 turbed mass of the Indian Peninsula, anil Mr. E. D. Oldham,^ with this view, 

 has also calculated the effect of such a wedge of alluvial material of low specific 

 gravity, coming to the conclusion that the rapid change in deflection, on 

 passing from the Lower Himalaya southward towards the peninsula, can 

 mainly be explained by the deficiency of mass in the alluvium itself. 



It is obvious that, before seeking for any unusual cause for the gravity 

 anomalies, we ought to take into account the effect of this large body of 

 alluvium which lies along the southern foot of the range. It is, however, by no 

 means certain that a thick mass of alluvial material, accumulated slowly and 

 saturated with water largely charged with carbonate of lime, would have a specific 

 gravity so appreciably lower than that of the rocks now exposed in the main 

 mass of the Himalaya as to account for the residual anomalies. Some of the 

 apparent deficiency in gravity is due to this body of alluvium, but it will only 

 be after critical examination of the data and more precise computation that we 

 shall be in a position to say if there is still room to entertain Colonel Burrard's 

 very interesting hypothesis. 



By bringing together the geological and geodetic results we notice five 

 roughly parallel bands stretching across Northern India. There is (1) a band 

 of abnormal high gravity lying about 1-50 miles from the foot of the mountains, 

 detected by the plumb-line and pendulum ; (2) the great depression filled by 

 the Gangetic alluvium ; (3) the continuous band of Tertiary rock, forming the 

 sub-Himalaya, and separated by a great boundary overthrust from (4) the main 



^ Rec. Geol. Surv. Ind., vol. xliii, pt. ii, p. 138, 1913. 



^ Rec. Surv. Ind., vol. v, p. 1. 



^ Proc. Roy. Soc, ser. A, vol. xc, p. 32, 1914. 



