The ages of the trees growing upon the Marietta inclosures do not however enable a date to be estimated for the construction of such works as those at Newark, for the shapes at Marietta are irregular and, according to the survey, do not appear to have been laid down with geometrical accuracy. It is therefore probable that the Newark inclosures were made at some later period. The fact of a ferule of silver and a plate of iron having been placed with the skeletons in the burial mound at Circleville leads to the conclusion that the tumulus like the one at Marietta was raised since the time of the arrival of the Europeans. On account of its having been placed within the inclosure it was originally conjectured that it belonged to the same people that formed the surrounding embankment, but the evidence is not sufficient to establish the correctness of a theory of such importance. It is difficult to understand what could have been the object of the Indians in constructing large earthworks in the shapes of squares and circles. Various theories have been advanced upon the subject, but nothing that can be considered satisfactory has yet been ascertained. Upon an examination of the plans it naturally occurs to the mind to endeavour to form an opinion as to the reasons which led to the adoption of these particular forms. It is probable that these types of inclosures would be convenient for the habits and purposes of an Indian tribe during peace, and that they afforded protection in war. The square inclosures may have been intended to contain the village, the dwellings of the chiefs, and the council house. The circles, with their single opening for approach, which could be strongly guarded, would in that case be the strongholds in which, during hostilities, would be placed the women and children. A circular fort, such as that at Newark, would, under the usual conditions of Indian warfare, be practically impregnable. In the event of the outer square being captured it would have a sufficient area to give the space that would be wanted for the defending tribe. The antiquity of these works is a problem that does not possess all the elements that are required for its solution. But in consequence of the exceptional system of construction certain inferences can be determined. It may be assumed that the geometrically shaped inclosures could not have been planned by Indians, and that therefore the square, circular and octagonal works, were constructed at some period subsequent to the landing of the Spaniards in Florida, in the beginning of the sixteenth century.[30] If these embankments were raised before that period, it would be almost necessary to admit, that white men possessing a knowledge of mathematics were living amongst the Indians before the discovery of America by Columbus in 1492.