The people of the United States, though much to be commended because of their prudence and forethought in laying out their modern towns and cities along the various water courses, which serve as the different highways of commerce, have by no means shown a superior sagacity in that respect to the Mound-builders, whose great centres of population are now mostly occupied, or are encroached upon by the modern cities. We may with safety assert that the population about Newark, and Xenia, and Mound City, was far above what it is now. The country about Dayton, Miamisburg, Oxford, Hamilton and Marietta was, undoubtedly, in the days of the Mound-builders moving with a greater mass of human beings than it can boast of to-day. And if those peaceable and industrious inhabitants were as numerous as their remains indicate, what must have been the strength of those invading hordes who caused their downfall and perhaps wiped out forever every living representative of that ancient race, who could leave no more lasting memorial of their existence and struggles than those mysterious mounds which have given them their name. ANTIQUITY OF THE MOUND-BUILDERS.--Upon this point there are many theories, some regarding them as the earliest of the Indian tribes. Others give them a very great age and claim them to belong to preadamite man. By far the greater number of archaeologists, however, place their existence at about 2,000 years ago. In favor of the latter view we may call as evidence the present forest trees, which, though of great age, still flourish on some of the ancient remains. On one of the mounds at Marietta, Ohio, there stood a gigantic tree, which, when cut down, displayed 800 rings of annual growth. In many other places, trees of the age of 750 years have been cut, and underneath them evidences of previous forests found. One tree 750 years old was found to have underneath it, on the walls of one of the forts in Ohio, the cast of another tree of equal size, which would carry us back at least 1,500 years since those trees began to grow on those deserted walls of that ancient fortification. We have some data in the vegetable accumulations in the ancient mining shafts near Lake Superior, as well as in the vegetable and other matter deposited in the numerous pits and trenches found among the works. Though these evidences cannot give the exact time of their accumulation, yet they give it approximately, by comparison with similar recent deposits.