is the great circular structure previously referred to. Its actual form is an ellipse; the different diameters of which are 1250 feet and 1150 feet respectively; and it encloses an area of upwards of 30 acres. At the entrance the enclosing embankment curves outward on either side for a distance of 100 feet, leaving a level way between the ditches, 80 feet wide, and at this point it measures about 30 feet from the bottom of the ditch to the summit. The area of the enclosure is almost perfectly level, so that during rain-floods the water stands at a uniform height nearly to the edge of the ditch. The skulls of the Mound-Builders have been appealed to for indications of the intellectual capacity of the ancient race; but mounds and earthworks were habitually resorted to at long subsequent dates as favourite places of interment; so that skulls derived from modern graves are ascribed to the ancient race; and much difficulty has been found in agreeing on a typical mound skull. Even after making allowance for modifications due to artificial malformation, and eliminating those derived from superficial interments, a very noticeable diversity is found in the comparatively few undoubtedly genuine mound skulls, which may lend some countenance to the idea of the presence of two essentially distinct races among the ancient settlers in the Ohio valley.[26] It seems to accord with the unmistakable traces of intellectual progress of a kind foreign to the attainments of any known race of the North American continent, thus found in association with arts and methods of work not greatly in advance of those of the Indian savage. The only satisfactory solution of the problem seems to present itself in the assumption of the existence among them of a theocratic order, like the priests of ancient Egypt, the Brahmins of India, or the Incas of Peru, under whom the vanished race of the Ohio valley—Tallegewi, Muskogees, Natchez, Alleghans, or other American aborigines,—executed their vast geometrical earthworks with such mathematical accuracy.