The chief characteristic of the Mound Builders was that from which they derived their name--the creation of great structures of earth or stone, not unlike the pyramids of Mexico and Egypt. Between Alton and East St. Louis is the great mound of Cahokia, which may be selected as a type of their works: it rises ninety-seven feet high, while its square sides are 700 and 500 feet respectively. There was a terrace on the south side 160 by 300 feet, reached by a graded way; the summit of the pyramid is flattened, affording a platform 200 by 450 feet. It will thus be seen that the area covered by the mound of Cahokia is about as large as that of the greatest pyramid of Egypt, Cheops, although its height is much less. The number of monuments left by the Mound Builders is extraordinarily great. In Ohio alone there are more than ten thousand tumuli, and from one thousand to fifteen hundred enclosures. Their mounds were not cones but four-sided pyramids--their sides, like those of the Egyptian pyramids, corresponding with the cardinal points. (Foster's "Prehistoric Races," p. 112.) The Mound Builders had attained a considerable degree of civilization; they were able to form, in the construction of their works, perfect circles and perfect squares of great accuracy, carried over the varying surface of the country. One large enclosure comprises exactly forty acres. At Hopetown, Ohio, are two walled figures--one a square, the other a circle--each containing precisely twenty acres. They must have possessed regular scales of measurement, and the means of determining angles and of computing the area to be enclosed by the square and the circle, so that the space enclosed by each might exactly correspond. "The most skilful engineer of this day would find it difficult," says Mr. Squier, "without the aid of instruments, to lay down an accurate square of the great dimensions above represented, measuring, as they do, more than four-fifths of a mile in circumference.... But we not only find accurate squares and perfect circles, but also, as we have seen, octagons of great dimensions." They also possessed an accurate system of weights; bracelets of copper on the arms of a skeleton have been found to be of uniform size, measuring each two and nine-tenth inches, and each weighing precisely four ounces. They built great military works surrounded by walls and ditches, with artificial lakes in the centre to supply water. One work, Fort Ancient, on the Little Miami River, Ohio, has a circuit of between four and five miles; the embankment was twenty feet high; the fort could have held a garrison of sixty thousand men with their families and provisions. Not only do we find pyramidal structures of earth in the Mississippi Valley very much like the pyramids of Egypt, Mexico, and Peru, but a very singular structure is repeated in Ohio and Peru: I refer to the double walls or prolonged pyramids, if I may coin an expression, shown in the cut page 375.