September 3, 19 14] 



NATURE 



»5 



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 over- 

 thrust 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 penin- 

 sula, the depression between being filled up gradually 

 bv the Gangetic alluvium. So far as the purely strati- 

 graphical 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 sedi- 

 ments forming a fairly complete record from Palaeo- 

 zoic 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 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 directed attention to some conclusions which, from 

 our present geological knowledge, appear to be 

 stiange and improbable in Colonel Burrard's con- 

 clusions, and he also offers alternative explanations 

 for the admitted geodetic facts. Mr. Hayden sug- 

 gests, for instance, that the depth of isostatic com- 

 pensation 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 

 Lpper 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 undisturbed 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 de- 

 ficiency of mass in the alluvium itself. 



It is obvious that, before seeking for any unusual 



^ '"Rec. Geol Surv. Ind.," vol. xliii., part 2, p. 138, 1913. 

 * ■' Records of the Survey of India," vol. v., p. 1. 

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



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 pre- 

 cise computation that we shall be in a position to say 

 if there is still room to entertain Colonel Burrard's 

 very interesting hypothesis. 



B}' bringing together the geological and geodetic 

 results we notice five roughly parallel bands stretching 

 across northern India. There is (i) a band of ab- 

 normal 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 Ter- 

 tiary- rock, forming the sub-Himalaja, and separated 

 by a great boundary overthrust from (4) the main 

 mass of the Outer and Central Himalaya of old un- 

 fossiliferous rock, with the snow-covered crA'stalline 

 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 

 scarcely be without generic relationship one to another 

 is indicated by the geological history of the area. 

 Until 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 great out- 

 welling 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 5000 ft. 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 eastw^ards to a little 

 bevond the point at which the Ganges now emerges 

 from the hills. Then followed the folding move- 

 ments that culminated in the present Himalayan 

 range, the elevation developing first on the Bengal 

 side, and extending rapidly to the north-west until 

 the folds extended in a great arc for some 1400 miles 

 from south-east to north-west. 



New streams developed on the southern face of the 

 now rising mass, and although the arm of the sea 

 that existed in early Tertiary times became choked 

 with silt, the process of subsidence continued, and the 

 gradually subsiding depression at the foot of the hills 

 as fast as it developed became filled with silt, sand, 

 gravel, and boulders in increasing quantities as the 

 hills became mountains and the range finally reached 

 its present dimensions, surpassing in size all other 

 features of the kind on the face of the globe. 



Now, it is important to remember that for ages 

 before the great outburst of Deccan Trap occurred 

 there was a continual unloading of Gondwanaland, 

 and a continual consequent overloading of the ocean 

 beds immediately to the north; that this process went 

 on with a gradual rise on one side and a gradual 

 depression on the other ; and that somewhere near and 

 parallel to the boundary line the crust must have been 

 undergoing stresses which resulted in strain, and, as 

 I suggest, the development of those fissures that let 

 loose the floods of Deccan Trap and brought to an 

 end the delicate isostatic balance. 



XO. 2340, VOL. 94] 



