At the present time a large proportion of the world's supply of tungsten ores comes from secondary (detrital) deposits of various kinds, formed by the normal denudation and redeposition of primary ores exposed at the surface to the agents of weathering and transport. It is impossible to form any idea of what fraction of the world's output actually comes from these sources, since the published statistics do not draw any distinctions in this respect', but the amount is undoubtedly large. Although of such great economic value, the secondary deposits do not show any features of special interest, and a lengthy description is unnecessary.