The iron ores of the Lake Superior region are the result of the action of waters from the surface on so-called iron formations or jaspers. Here again it was at first supposed that the enrichment was related to the present erosion surface; but upon further studies the fact was disclosed that the concentration of the ores took place in the period between the deposition of Keweenawan and Cambrian rocks, and thus a new light was thrown on the possibilities as to depth and distribution of the ores. The old pre-Cambrian surface, with reference to which the concentration took place, can be followed with some precision beneath the present surface. This makes it possible to forecast a quite different depth and distribution of the ores from that which might be inferred from present surface conditions. Present surface conditions, of low relief, considerable humidity, and with the water table usually not more than 100 feet from the surface, do not promise ore deposits at great depth. The erosion which formed the old pre-Cambrian surface, however, started on a country of great relief and semi-arid climate, conditions which favored deep penetration of the surface waters which concentrated the ores. The iron ores of eastern Cuba are formed by the weathering of a serpentine rock on an elevated plateau of low relief, where the sluggish streams are unable rapidly to carry off the products of weathering. Where streams have cut into this plateau and where the plateau breaks down with sharp slopes to the ocean, erosion has removed the products of weathering, and therefore the iron ore. An important element, then, in iron ore exploration in this country is the location of regions of slight erosion in the serpentine area. One of the largest discoveries was made purely on a topographic basis. It was inferred merely from a study of topography that a certain large unexplored area ought to carry iron ore. Subsequent work in the thick and almost impenetrable jungle disclosed it. Bauxite deposits in several parts of the world require somewhat similar conditions of concentration, and a study of the physiographic features is an important factor in their location and interpretation. A physiographic problem of another sort is the determination of the conditions surrounding the origin of sedimentary ores. Certain mineral deposits, like the "Clinton" iron ores, the copper ores in the "Red Beds" of southwestern United States and in the Mansfield slates of Germany, many salt deposits, and almost the entire group of placer deposits of gold, tin, and other metals, are the result of sedimentation, from waters which derived their materials from the erosion of the land surface. It is sometimes possible from the study of these deposits to discover the position and configuration of the shore line, the depth of water, and the probable continuity and extent of the deposits. Similar questions are met in the study of coal and oil.