_Antiquity._--There are methods of determining the antiquity of these mounds. Mr. E. G. Squier has pointed out three facts which go to prove that they belong to a distant period. 1. None of these ancient works occur on the lowest formed of the river terraces, which mark the subsidence of the streams. As these works are raised on all the others, it follows that the lowest terrace has been formed since the works were erected. The streams generally form four terraces, and the period marked by the lowest must be the longest because the excavating power of such streams grows less as the channels grow deeper. 2. The skeletons of the Mound-Builders are found in a condition of extreme decay. Only one or two skeletons have been recovered in a condition suitable for intelligent examination. The circumstances attending their burial were unusually favorable for preserving them. The earth around them has invariably been found wonderfully compact and dry; and yet, when exhumed, they have been in a decomposed and crumbling condition. 3. Their great age is shown by their relation to the primeval forests. As the Mound-Builders were a settled agricultural people, their enclosures and fields were cleared of trees, and remained so until deserted. When discovered by the Europeans these enclosures were covered by gigantic trees, some of them eight hundred years old. The trees which first made their appearance were not the regular forest trees. When the first trees that got possession of the soil had died away, they were supplanted, in many cases, by other kinds, till at last, after a great number of centuries, that remarkable diversity of species characteristic of North America would be established.[89] Dr. Buchner assigns to them an antiquity of from seven thousand to ten thousand years.[90] Fort Shelby, in Orleans county, New York, was carefully examined by Frank H. Cushing, the archæologist. The fort was found to be composed of two parallel circular walls, with a gateway in each. The gateway in the outer wall fronted a peat-bog, the shore of which was some ten feet distant. Within the enclosure he found small, flat, notched stones, used for sinking fishing-nets. Into the bog he sank a shaft to the depth of seven feet, not far from the shore. At the bottom of the shaft he found the shells of living species of shell-fish. The natural surroundings show that this fort was built when the peat-bog was a lake. This is further confirmed by the fact that all ancient works are erected near a permanent supply of water. The nearest permanent supply of water is Oak Orchard Creek, one and one-half mile distant. The formation of this peat would require not less than four thousand years, and more probably twice that number. The Mound-Builders must have remained a very long time. These works were formed gradually, and the population extended slowly toward the North. Their corn-fields, by their raised condition, show many successive years of usage.