But, while the Mound-Builders are essentially prehistoric, according to all New World chronology, there is nothing in the disclosures hitherto made calculated to suggest for them an extremely remote era. The marvellous traces of geometrical skill in their great earthworks, more than anything else, separate them from every known race north of Mexico. The indications of antiquity in the mines of Lake Superior, and the mounds of Ohio, suggest no such enormous intervals of time as perplex us in attempting to deal with the relics of the caves and river-valleys of Europe. The refilled trenches on the barren rocks of Isle Royale manifestly demand centuries for the slow accumulation of sufficient soil and vegetable matter to refill the excavations. Dr. Hildreth ascribes eight hundred years of growth to a tree felled on one of the mounds at Marietta; and other trustworthy authorities, including Messrs. Squier and Davis, furnish similar evidence for lesser periods of four, five, and six centuries. The longest term thus indicated would be little enough for the filling up of the deserted trenches of Isle Royal. But however far back we carry the era of the Mound-Builders, the chief change which the regions occupied by them have since undergone, is the clothing of their valleys, and the earthworks erected there, with the forests which help us to some partial guess at the intervening centuries since their disappearance. The animal remains hitherto found in their mounds are those of the existing species of deer, bears, wolves, and other fauna, not even now wholly extirpated from Ohio; and while their ingenious sculptures prove that they were familiar with a more southern, and even a South-American tropical fauna: nothing has yet been discovered to connect them with an extinct, much less a fossil mammalia, such as the mastodon. The probability rather is that the ruins of Clark’s Work, or Fort Ancient, may match in antiquity with those of England’s Norman keeps, and even that their builders may have lingered on into centuries nearer the age of Columbus. The Zuñi, Moquis, Pimos, and other tribes of New Mexico, have left curious evidences of a people of gentle skill in agriculture, in ceramic art, and above all, in architecture, beyond anything pertaining to the northern Indians, or even in some respects to the Mound-Builders. But there still remains the distinct and perplexing element of a people so partially civilised, and comparatively rude; yet able to construct squares, circles, ellipses, and other geometrical figures on a seale which would tax the skill of many a well-trained civil engineer of the present day.