In some cases erosion has followed so
slowly as to leave large zones of secondary enrichment. In other cases
erosion has followed up so closely after the processes of secondary
enrichment as to remove from the surface important parts of the
secondarily enriched deposits.

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 interpretat