Much uncertainty continues to shroud the design of these mounds. Some were for military defence, others for burial places, others for lookout stations, others apparently for religious uses. Still others, it is supposed, formed parts of human dwellings. That they proceeded from intelligence and reflection is clear. Usually, whether they are squares or circles, their construction betrays nice, mathematical exactness, unattainable save by the use of instruments. Many constitute effigies--of birds, fishes, quadrupeds, men. In Wisconsin is a mound 135 feet long and well proportioned, much resembling an elephant; in Adams County, 0., a gracefully curved serpent, 1,000 feet long, with jaws agape as if to swallow an egg-shaped figure in front; in Granville, in the same State, one in the form of a huge crocodile; in Greenup County, Ky., an image of a bear, which seems leaning forward in an attitude of observation, measuring 53 feet from the top of the back to the end of the foreleg, and 105-1/2 feet from the tip of the nose to the rear of the hind foot. [Illustration: Big Elephant Mound, Wisconsin.] The sites of towns and cities were artfully selected, near navigable rivers and their confluences, as at Marietta, Cincinnati, and in Kentucky opposite the old mouth of the Scioto. Points for defence were chosen and fortified with scientific precision. The labor expended upon these multitudinous structures must have been enormous, implying a vast population and extensive social, economic, and civil organization. The Cahokia mound, opposite St. Louis, is 90 feet high and 900 feet long. The Mound-builders made elegant pottery, of various design and accurate shapes, worked bone and all sorts of stones, and even forged copper. There are signs that they understood smelting this metal. They certainly mined it in large quantities, and carried it down the Mississippi hundreds of miles from its source on Lake Superior. They must have been masters of river navigation, but their mode of conveying vast burdens overland, destitute of efficient draft animals as they apparently were, we can hardly even conjecture. The Mound-builders, as we have said, were related to the antique populations of Mexico and Central America, and the most probable explanation of their departure from their Northern seats is that in face of pestilence, or of some overpowering human foe, they retreated to the Southwest, there to lay, under better auspices, the foundations of new states, and to develop that higher civilization whose relics, too little known, astound the student of the past, as greatly as do the stupendous pillars of Carnac or the grotesque animal figures of Khorsabad and Nimrud.