In the details of their works, and in manufactured articles taken from the mounds, there is evidence of considerable civilization. For instance, it has been ascertained that the circular inclosures are perfect circles, and the square inclosures perfect squares. They were constructed with a geometrical precision which implies a kind of knowledge in the builders that may be called scientific. Figures 13, 14, 15, 16 show some of the more important works of the Mound-Builders, chiefly in Ohio. Relics of art have been dug from some of the mounds, consisting of a considerable variety of ornaments and implements, made of copper, silver, obsidian, porphyry, and greenstone, finely wrought. There are axes, single and double; adzes, chisels, drills or gravers, lance-heads, knives, bracelets, pendants, beads, and the like, made of copper. There are articles of pottery, elegantly designed and finished; ornaments made of silver, bone, mica from the Alleghanies, and shells from the Gulf of Mexico. [Illustration: Fig. 16.--Rectangular Work, Randolph County, Indiana.] The articles made of stone show fine workmanship; some of them are elaborately carved. Tools of some very hard material must have been required to work the porphyry in this manner. Obsidian is a volcanic product largely used by the ancient Mexicans and Peruvians for arms and cutting instruments. It is found in its natural state nowhere nearer the Mississippi Valley than the Mexican mountains of Cerro Gordo. There appears to be evidence that the Mound-Builders had the art of spinning and weaving, for cloth has been found among their remains. At the meeting of the International Congress of Pre-Historic Archæology held at Norwich, England, in 1868, one of the speakers stated this fact as follows: “Fragments of charred cloth made of spun fibres have been found in the mounds. A specimen of such cloth, taken from a mound in Butler County, Ohio, is in Blackmore Museum, Salisbury. In the same collection are several lumps of burnt clay which formed part of the ‘altar,’ so called, in a mound in Ross County, Ohio: to this clay a few charred threads are still attached.” Figures 17 and 18 represent specimens of vases taken from the mounds. [Illustration: Figs. 17, 18.--Vases from the Mounds.]