of lead or galena, containing also a considerable proportion of zinc. The objective of the Chinese smelters seems to have been the silver, for the lead slag was left in heaps to be utilized ultimately by the Company at present working the mine. Between 17½ and 54 ounces of silver to the ton are obtainable from the ore. The ores owe their present home to the compressional movement mentioned in the previous chapter. This movement has produced a zone of fracture, displacement and general disturbance along a north-to-south line passing through Bawdwin. Into this zone of broken rock mineral-bearing solutions have percolated and left their valuable deposits, especially within some decomposed and very ancient volcanic ash beds. The output of refined lead from these mines in 1921 was 33,717 tons, of fine silver 3,555,021 ounces.
Similar old silver-lead mines worked by the Chinese are described as occurring at Bhamo. Ore of this nature is known in the Amherst and Mergui districts, in the Southern Shan States, at Mt Pima in the Yamèthin district, and in the Yônzalin valley near the Salween.
The most valuable mineral Burma possesses is Petroleum, which is found in the Lower Tertiary beds of the Irrawaddy basin on the one side and of the Arakan coast on the other. These beds are, in fact, the "gulf" deposits mentioned in the chapter on Geology, and the formation of petroleum within them, perhaps from some form of vegetation, seems to be connected with the conditions produced by the silting up of the gulf. The desiccation caused the precipitation of gypsum and other sea-water salts, and the saline conditions established are thought by some to have directed the changes taking place in the decomposing vegetation of that period, and to have induced the formation of petroleum instead of lignite or coal. Whatever the original material was, petroleum accumulated in the porous sands of the Miocene period, and was prevented from escaping at the surface by thick caps of
