The execution of the outlines of correct circles inclosing spaces of nearly thirty acres, presents still greater difficulties. It would have required a specially trained mind to form the conception of a circumference having an imaginary point within, from which all lines drawn to it would be equal. But the figure which would have been absolutely impracticable to construct without proper surveying appliances for making accurate measurements, and fixing the true angles, is that of the octagon. Even under the most favourable circumstances, with the help of suitable instruments, it would have required much skill and calculation to trace a true octagon whose embankments contained within them an area exceeding forty acres. It is difficult to suppose that an accurately designed work of this shape and magnitude could have been planned by Indians, or that the construction of a figure so essentially scientific and unusual, could have been originated by them. It is therefore possible to conclude, that, the geometrical earthworks in Ohio may have been raised by native tribes, acting under the direction of European surveyors, or of men who had received a mathematical education. Considerations upon the subject of the race and capacity of the builders, have been to some extent complicated by the reports that were made concerning the ages of the trees that grew upon and within the ancient ramparts at Marietta. In the letter of Dr. Hildreth, quoted in the previous chapter, it is clearly stated that trees were growing there which were from four to five hundred years old. As this evidence is very important it is desirable to examine it with attention. This statement was made when he was attempting to fix an approximate date for the age of a burial mound which was placed near the fortifications. It was at the bottom of this mound that were discovered the ornaments of a sword belt and scabbard. These ornaments were made of copper and plated with silver, and must therefore have been of European manufacture. The calculation of the age of the trees is probably based upon the results of an examination that took place shortly after the settlement of Marietta during the latter part of the eighteenth century. It was at that time decided by several of the inhabitants to fell some of the largest and oldest trees then growing within the earthworks, and ascertain their ages by counting the number of annual rings contained in them. The operations were executed in the presence of Governor St. Clair and the Rev. Dr. Cutler. Several of the trees were found to have between three hundred and four hundred circles. One tree was carefully examined and Dr. Cutler stated that it contained at least four hundred and sixty-three circles.[29] As nothing can be more conclusive as a proof of age than the number of rings found in any tree growing beyond the tropics, this evidence establishes an antiquity for these embankments earlier than the middle of the fourteenth century.