PRESIDENTIAL ADDRESS. 355 
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 coast-line 
must have been very much further to the south before the Himalayan folding 
began. 
Representing 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 different 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 three or four 
miles deep near its northern margin, thinning out gradually towards the undis- 
turbed mass of the Indian peninsula, and Mr. R. 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 residaal 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 150 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 
mass of the Outer and Central Himalaya of old unfossiliferous rock, with the 
snow-covered crystalline peaks flanked on the north by (5) the Tibetan basin of 
highly fossiliferous rocks formed in the great Eurasian mediterranean ocean 
that persisted up to nearly the end of Mesozoic times. 
That these leading features in North India can hardly be without genetic 
relationship one to another is indicated by the geological history of the area. 
Till nearly the end of the Mesozoic era the line of crystalline, snow-covered 
peaks now forming the Central Himalaya was not far from the shore-line 
between Gondwanaland, stretching away to the south, and Tethys, the great 
Eurasian ocean. Near the end of Mesozoic times there commenced the preat 
outwelling of the Deccan Trap, the remains of which, after geological ages of 
erosion, still cover an area of 200,000 square miles, with a thickness in places of 
nearly 5,000 feet. Immediately after the outflow of this body of basic lava, 
greater in mass than any known eruption of the kind, the ocean flowed into 
North-West India and projected an arm eastwards to a little beyond the point 
°° Ree. Geol. Surv. Ind., vol. xliii. part 2, p. 138, 1913. 
°° Records of the Survey of India, vol. v. p. 1. 
27 Proc. Roy. Soc., Series A, vol. 90, p. 32, 1914. 
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