If mounds were the only works remaining of these ancient people, we would not take so great an interest in them, and, as it is, we are not to suppose that all the mounds are the works of those people we call the Mound Builders. Recent investigation and historical evidence unite in showing that some comparatively recent Indian tribes formed and used mound structures. Early explorers have left abundant testimony to show that in many cases the Indians resorted to mound-burial. Thus, it seems that it was the custom of the Iroquois every eighth or tenth year, or whenever about to abandon a locality, to gather together the bones of their dead and rear over them a mound. To this custom, which was not confined to the Iroquois, are doubtless to be ascribed the barrows and bone mounds which have been found in such numbers in various parts of the country.17 Although it is well to bear these facts in mind, yet it is not doubted that the larger number, and especially the more massive ones, were erected by the same people who built the other mysterious works, and so it is necessary that they be carefully studied. Altar Mound and Plan and Section of Altar. In the valley of the Ohio there have been found a class of mounds known as Altar Mounds. These, it should be stated, nearly always occur in or near inclosures. This cut gives us a good idea of mounds of this kind. Near the top is seen an instance of what is called “intrusive” burial. After the mound was completed it had been dug into and a body buried near the surface. This burial was evidently later in time, and had no connection with the purpose for which the mound was originally built. We also notice in this mound the different layers of which it was composed. These layers are of gravel, earth, and sand, the latter being only a few inches thick. Mounds made in this manner are called stratified mounds, and all altar mounds are probably of this kind. The lines of stratification have been described as curving so as to correspond with the shape of the mound, and such we are told is the general rule.18