The extraordinary number of the mounds implies a long period during which a settled agricultural population, considerably advanced in the industrial arts, occupied the fertile valleys or the alluvial plains in the basin of the Mississippi, covered, since then, with vast forests, and tenanted by wild hunters without any traditionary connexion with their more civilised predecessors. The epoch when this people flourished, or the adverse circumstances which swept them away, are all equally unknown; but the age and nature of the trees found growing on some of their earthworks afford at least some data for estimating the minimum of time which must have passed since the mounds were abandoned. Trunks, displaying eight hundred rings of annual growth, have been cut down from them, and several generations of trees must have lived and died before the mounds could have been overspread with that variety of species which they supported when the white man first set foot in the valley of the Ohio. In a memoir on this subject, General Harrison, who was skilled in woodcraft, observes that ‘beyond all doubt no trees were allowed to grow so long as the earthworks were in use; and when they were forsaken, the ground, like all newly cleared ground in Ohio, would for a time be monopolised by one or two species of tree, such as the yellow locust and the black or white walnut. When the individuals which were the first to get possession of the ground had died out one after the other, they would in many cases, instead of being replaced by the same species, be succeeded (by virtue of the law which makes a rotation of crops profitable in agriculture) by other kinds, till at last, after a great number of centuries (several thousand years, perhaps), that remarkable diversity of species characteristic of North America, and far exceeding what is seen in European forests, would be established.’