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SCIENCE 



[N. S. Vol. XL. No. 1033 



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 south- 

 ward 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, how- 

 ever, 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 spe- 

 cific gravity so appreciably lower than that 

 of the rocks now exposed in the main mass 

 of the Himalaya as to account for the resi- 

 dual anomalies. Some of the apparent de- 

 ficiency 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 hy- 

 pothesis. 



By bringing together the geological and 

 geodetic results we notice five roughly par- 

 allel 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 the (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 rela- 

 tionship 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 form- 

 ing the Central Himalaya was not far from 

 the shore-line between G-ondwanaland, 

 stretching away to the south, and Tethys, 

 the great Eurasian ocean. Near the end of 

 Mesozoic times there commenced the great 

 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 Northwest India and 

 projected an arm eastwards to a little be- 

 yond the point at which the Ganges now 

 emerges from the hills. Then followed the 

 folding movements that culminated in the 

 present Himalayan range, the elevation de- 

 veloping first on the Bengal side, and ex- 

 tending rapidly to the northwest until the 

 folds extended in a great are for some 

 1,400 miles from southeast to northwest. 



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 be- 

 came filled with silt, sand, gravel and 

 boulders in increasing quantities as the 

 hills became mountains and the range 

 finally reached its present dimensions, sur- 

 passing 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 Dec- 



