The process of working the ancient mines seems to be tolerably clearly indicated by the discoveries referred to. The soil having been removed by means of wooden spades, doubtless with the aid of copper tools to break up the solid earth and clay: remains of charcoal, met with in numerous instances on the surface of the rock, show that fire was an important agent for overcoming the cohesion between the copper and its matrix. Before the introduction of gunpowder fire was universally employed in excavating rock; and where fuel abounds, as in the old Harz and Altenberg mining districts of Europe, it is even now found to be quite as economical in destroying siliceous rocks. Stone hammers or mauls were next employed to break up the metalliferous rock. These have been found in immense numbers on different mining sites. Mr. Knapp obtained in one locality upwards of ten cart-loads; and I was shown a well at Ontonagon constructed almost entirely out of stone hammers, obtained from ancient workings in the immediate vicinity. Many of these are mere water-worn boulders of greenstone or porphyry, roughly chipped at the centre, so as to admit of their being secured by a withe around them. But others are well-finished, with a single or double groove for attaching the handle by which they were wielded. They weigh from ten to forty pounds; but many are broken, and some of the specimens I saw were worn and fractured from frequent use. The extent to which co-operation was carried on by the miners, with the imperfect means at their command, is illustrated by the objects recovered on exploring one of their trenches, on a hill to the south of the Copper Falls mines. On removing the accumulations from the excavation, stone axes of large size made of greenstone, and shaped to receive withe-handles, and some large round greenstone masses that had apparently been used for battering-rams, were found. “They had round holes bored in them to the depth of several inches, which seemed to have been designed for wooden plugs to which withe-handles might be attached, so that several men could swing them with sufficient force to break the rock and the projecting masses of copper. Some of them were broken, and some of the projecting ends of rock exhibited marks of having been battered in the manner here suggested.”[74]