the Himalaya is a simple curve convex towards the direction of the compressional force, the Naga Hills and Arakan Yoma with the Andamans and Nicobars form a well-marked double S-shaped curve.
It is difficult to say precisely when the Naga Hills and Arakan Yoma first began to appear from beneath the waters of the old Bay of Bengal, but the Meghna and ChindwinIrrawaddy gulfs seem to have been separated off from each other early in the Tertiary period; in these two gulfs were deposited the oil-bearing sediments which to-day are being tapped at various spots along the Arakan coast and at different localities in the Irrawaddy and Chindwin valleys. The movement from the east corrugated the floors of the gulfs and caused their silting up. A river in each case succeeded them and its sediments also were folded by the continued movement. This east-to-west movement is the key to all the present topography of Burma, and is the reason for the north-to-south sigmoidal direction of its mountain ranges and valleys. The Andaman and Nicobar Islands are an extension of the Arakan Yoma and are connected with it by a submarine shelf or ridge. The Chindwin river is thought to have been continuous at the end of the Tertiary period with the Tsanpo or Upper Brahmaputra, which was subsequently captured by the Assam Brahmaputra.
At the close of the "gulfs" period several volcanoes made their appearance, chief among which was the mysterious and legend-haunted hill of Mt Popa, which rises in lofty solitude from the plains of Myingyan. Its earliest lava flows were covered by river sediments, but subsequent ejections of lavas and ashes have piled a cone nearly 5000 feet above sea-level. Flows of a different type occurred from other vents in the immediate neighbourhood, and several craters, thought to have been produced by gigantic explosions, are to be seen along the banks of the Lower Chindwin.
