The ore bodies of this class are formed by the concentration of the metals disseminated in the rocks, through solution and deposition by surface waters. Such deposits are made not far from the surface, to which they show a definite relation, and disappear downward. The most abundant ores of this class are those of iron and are exemplified in the famous Lake Superior region.

Many of the ores of lead and zinc, which also occur in veins, seem to be referable to this class, though the bodies have no definite relation in form to the surface. Such ores occur in limestones, in crevices, along joint or bedding-planes, in cavities, or by replacement of the country rock, and appear to have no connection with any fissures rising from great depths, nor with intrusive masses of igneous rock. The mode of formation of these ore bodies has been the subject of much discussion and is not yet entirely clear, but in the case of the upper Mississippi Valley, for example, it is very generally believed that the deposition has been accomplished by descending and circulating waters from the surface which have dissolved and concentrated the metallic sulphides originally disseminated thinly through the limestones. The disseminated sulphides are supposed to have been deposited in the limestones at the time the latter were accumulating in a great inland sea, being brought in solution from the land.

There is ground for believing that lead is but one member of a series of radio-active elements, and, if this is true, we shall be unable to determine which of these elements was the one actually deposited in that ancient sea.