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58 probably the whole of the Naga Hills and Arakan Yoma, as well as the Andaman and Nicobar Islands, were covered by the sea which formed the ancient Bay of Bengal.

Two of the earth-movements mentioned above have now to be considered, one from the north and one from the east, both advancing towards the centre of the Indian peninsula. By the movement from the north the northern margin of the Gondwana continent was folded again and again into the mighty mass of the Himalaya, the Tibetan sea retreating westwards until central Asia became dry land. In front and along the foot of this mountain mass was a deep trough, which at first formed a long gulf, but which subsequently became filled up with river sediments, and now coincides with the Ganges valley and portions of the valleys of the Indus tributaries. There is some evidence to show that behind the mountain mass was another less pronounced trough separating the mountain range from the Tibetan table-land, and that this was also occupied successively by a gulf and a river. The movement from the east produced an almost exactly similar effect upon Burma. The Shan plateau may be looked upon as corresponding to the lofty Tibetan table-land, the Arakan Yoma and Naga Hills take the place of the Himalaya, the Chindwin-Irrawaddy valley is the smaller trough behind the mountain mass, and the Arakan coast and the valley of the Meghna river which originally stretched into Upper Assam represent the large trough in front. Each of the two troughs, as in the case of the Himalayan troughs, was occupied successively by a gulf and a river, the rivers being the Chindwin-Irrawaddy (for the Chindwin was originally the head-waters of the main river) and the old Meghna. The point of interference between the two great movements just described was the north-eastern corner of Assam; here the mountain ranges of the two systems veer round till they confront one another in more or less parallel lines. One small difference between the two areas is that, whereas