When the Ohio Valley was first visited by Europeans it was covered by an unbroken forest, most of the trees being of great age and size; and it was manifest that several generations of great forest trees had preceded those found standing in the soil. The mounds and inclosures were discovered in this forest, with great trees growing in them. Eight hundred rings of annual growth were counted in the trunk of a tree mentioned by Sir Charles Lyell and others, which was found growing on a mound at Marietta. In the same way, successive generations of forest trees had grown over their extensive mining works near Lake Superior, and many of those works are still hidden in what seem to be primeval forests. General Harrison made the following suggestion in regard to the establishment of these forests in Ohio. When the individual trees that first got possession of the soil had died out one after another, they would, in many cases, be succeeded by other kinds, till at last, after a great number of centuries, that remarkable diversity of species characteristic of North America would be established. His suggestion, the result of practical observation and study, is not without reason. It is certain, in any case, that the period when these old constructions were deserted is so far back in the past, that sufficient time has since passed for the abandoned towns and fields to remain for years, and perhaps centuries, as waste places, pass through the transition from waste lands to the beginning of forest growths, and then be covered by several generations of such great forest trees as were cleared away to prepare the soil for the settlements, towns, and farms of our people. HOW LONG WERE THEY HERE? There are many indications to warrant the conclusion that the Mound-Builders occupied their principal seats in the Ohio and Mississippi Valleys during a very long period. If they came from the south, as appears evident, their settlements must have been extended up the valley gradually. After their first communities were established in the Gulf regions, considerable time must have elapsed before their advancing settlements were extended northward, through the intervening region, into the Valley of the Ohio. On the Ohio and in the valleys of its tributaries their settlements were very numerous, and evidently populous. The surprising abundance of their works in this region, which have been traced in our time, shows that they dwelt here in great numbers, and had no lack of industry.