part 3] OP THE INDUS, BB V II MAPI : THA, AX1) GA.NGES. 139 



Ganges, make it probable that the former was the original outlet of the drain- 

 age, and that the formation of the gap between the Rajmahal and the Garo 

 Hills, and of the Gangetic delta, is geologically of recent date.' 



To facilitate reference, 1 propose to call the ancient river whirl), 

 it is postulated, rose in Assam and flowed into the Arabian Sea, 

 the w Indobrahm.' Any particular portion of it will be referred to 

 under the name that it now bears and the area through which it 

 runs: for instance, the Assam Brahmaputra or the Kohat Indus. 



As the river in question was a result of earth-movements which 

 took place in Tertiary times, it will be advisable to begin by 

 glancing at the general topography of late -Cretaceous [ndia. 

 According to Dr. Hayden's description 1 there occurred towards the 

 close of the Cretaceous Period a great invasion of Tibet by the sea 

 ■of Western India. This sea, the Tethys, which was continuous 

 with the Mediterranean through Afghanistan, Baluchistan, Persia, 

 and North Africa, covered the southern and central parts of Tibet 

 and the northern part of the present Himalaya, and reached as far 

 as what is now the northern border of Sikkim. To the south of 

 this sea lay the remains of the great G-ondwaria continent, of 

 which peninsular India formed a part; south of that continent 

 was another sea, one arm of which projected northwards over parts 

 of Burma and Assam, while another indentation foreshadowed the 

 Arabian Sea. At the beginning of Eocene times the Himalayan 

 folding movement from the north, the Baluchistan movement from 

 the north-west, and the Shan movement in Burma from the east. 

 commenced to assume serious intensity.- The bard rigid body of 



1 S. G Bnrrard & H. H. Harden. ; A Sketch of the Geography & Geology 

 of the Himalaya Mountains & Tibet ' 1907-1908, p. 254. 



'-' It is assumed that isostatic tumefaction, which was primarily responsible 

 for the upheaval, enabled the pressure —due to whatever cause to plicate the 

 strata and play its part in the piling-up of the mountain-range. The direc- 

 tion of movement in a fold, as Mr. Oldham points out (Mem. Geol. Surv. Ind. 

 vol. xlii, 1917. p. [282"). is a purely relative matter, one side being thrust over 

 and the other under, in opposite directions. But the uniform consistency with 

 which all the mountain-arcs produced by this disturbance face the same way. 

 suggests that the folding movement, so far as the surface-rocks are concerned, 

 was stronger in the direction of their convexities than in the opposite 

 direction. The piling-up of the water in a circular ripple produced by the 

 dropping of a stone therein is the result not only of pressure from the centre, 

 but also of a resistance toward the centre. By suitable mechanical means a 

 similar ripple could be produced by a centripetal impulse. The former kind 

 of ripple is. however, so much more easily produced, and so much more com- 

 monly observed, that, when we meet with anything in Nature resembling a 

 ripple, it seems natural to imagine that it is the result of a centrifugal 

 impulse. Whatever their nature may be. it is convenient to assume thai 

 resultant movements, or Impulses (as they might more aptly be termed) took 

 place in that direction towards which the great mountain-arc-- are convex. 

 Even if we adopt this assumption, the direction of a movement is necessarily 

 referred to in this paper in a somewhat loose way. For instance, the move- 

 ment which produced the Afghanistan- Baluchistan- Persian arc system a^ a 

 whole came from the north ; but the component affecting Afghanistan an 1 

 Eastern Baluchistan may be considered as coming generally from tiic north- 

 west. Local components acted from the west, or from the north, or even 

 from the north-east. 



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