Remains of their mining works were first discovered in 1848 by Mr. S. O. Knapp, agent of the Minnesota Mining Company, and in 1849 they were described by Dr. Charles T. Jackson, in his geological report to the national government. Those described were found at the Minnesota mine, in upper Michigan, near Lake Superior. Their mining was chiefly surface work; that is to say, they worked the surface of the veins in open pits and trenches. At the Minnesota mine, the greatest depth of their excavations was thirty feet; and here, “not far below the bottom of a trough-like cavity, among a mass of leaves, sticks, and water, Mr. Knapp discovered a detached mass of copper weighing nearly six tons. It lay upon a cob-work of round logs or skids six or eight inches in diameter, the ends of which showed plainly the marks of a small axe or cutting tool about two and a half inches wide. They soon shriveled and decayed when exposed to the air. The mass of copper had been raised several feet, along the foot of the lode, on timbers, by means of wedges.” At this place was found a stone maul weighing thirty-six pounds, and also a copper maul or sledge weighing twenty-five pounds. Old trees showing 395 rings of annual growth stood in the débris, and “the fallen and decayed trunks of trees of a former generation were seen lying across the pits.” Figure 19 (opposite) presents a section of this mining shaft of the Mound-Builders: _a_ shows the mass of copper; _b_ the bottom of the shaft; _c_ the earth and débris which had been thrown out. The dark spots are masses of copper. The modern mining works are mostly confined to that part of the copper region known as Keweenaw Point. This is a projection of land extending into Lake Superior, and described as having the shape of an immense horn. It is about eighty miles in length, and, at the place where it joins the main land, about forty-five miles in width. All through this district, wherever modern miners have worked, remains of ancient mining works are abundant; and they are extensive on the adjacent island, known as Isle Royale. The area covered by the ancient works is larger than that which includes the modern mines, for they are known to exist in the dense forests of other districts, to which the modern mining has not yet been extended. [Illustration: Fig. 19.--Ancient Mining Shaft.]