Exploration.— The greater part of the area underlain by the copper-bearing formations of Lake Superior rise to an elevation of from 400 to 700 feet above the lalce. Several ridges stand out above this general plateau whose strrnmits show naked rock, and even in the lower ground there are some exposures of rock in place, but over a large area there is a heavy covering of edrift*— sand, gravel and boulders — which completely conceals the rock surface and renders difficult a successful search for mineral. When mining operations began on Keweenaw Point the attention of the early prospectors was attracted by grooved stone hammers, some of which weighed as much as 30 pounds, generally found on the ridges, in or by the side of shallow excavations that showed copper embedded in the rock. These traces of crude mining by a former generation evidently served as a guide to the early white miners in locating the most prominent deposits. The ridges naturally consist of the hardest rock — the massive trap beds that have resisted erosion better than the softer amygdaloids. By following these ridges the early prospectors located the fissure veins, and with them some quasi fissure veins that ran parallel with the formation and contained large masses of cop per, rough prongs of which projected above the rock surface. A *conformable vein* or an amygdaloid deposit thus located would be fol loived for miles, until perhaps it disappeared beneath a covering of drift. Sometimes a large block of cupriferous rock would be found, whose angularity indicated its probable nearness to the parent bed. Surmising the direction of the glacial movement the prospector would dig a series of trenches or costeans at right angles to the strike of the formation, on the side whence the glacial movement was supposed to have come. But here again in early days the fact was not recognized that Keweenaw Point had- been the scene of glacial movements from at least two directions, and thus in one well kricnvn instance explorations were vainly made in the wrong direction from the blocks of rich float-rock With the progress of geological investigation it was realized that the conglomerates inter bedded with the traps and representing old sea beaches were more or less continuous around the ancient basin. Their character, it is true, does not remain constant from point to point, for this depends in a great measure upon the rocks that lay behind them during the process of their formation. They furnished reliable aid to correlation, however, and could be fol lowed and recognized for long distances even across drift-covered areas. The lava beds, on the other hand, from their broad general simi larity, usually were not safe guides. In some cases, however, even these could be identified over long distances by some marked peculiarity. If a workable copper deposit was lcnown tor exist near one of these conglomerates or near a recognizable trap bed, the latter was traced out and a pit or trench was then dug at a known distance from it equivalent to the distance of that bed from the copper-bearing bed at the place where the latter was last seen, allowance being made for ascertainable changes in the dip of the formation between the two places. In this way the Baltic lode of the Champion Mine was located about three miles from the Baltic Mine, although in the intervening dis tance the strike of the formation had changed 34 degrees. By taking the great Greenstone ridge in Keweenaw County as a guide the posi tion of the northward extension of the Kear sarge amygdaloid bed was in a similar manner approximately ascertained about six miles from the nearest point at which it had been previously opened. While trenching was in progress to uncover this bed, the recognition of an isolated outcrop of its footwall —a very characteristic rock—pointed more exactly to the location of the amygdaloid. Exact calculations are not always possible, for the dip of the beds from younger to older may change—commonly does change— and can seldotn be determined for any one place, in advance of mining. The diamond drill is often an important aid in caset like the above, where the thickness of the drift covering makes trenching impossible or too expensive.
Development.— A mere deposit of mineral or an isolated mass is not necessarily synonymous with a amine,° as previously re marked. As now frequently understood, a °mine° is a deposit sufficiently exploited to afford reasonable hopes that it can be worked at a profit. To ascertain whether a native de posit of copper in Michigan will pay requires a good deal of time and the expenditure of a large amount of money. Shafts mutt be sunk, generally, in the lode to test the latter con tinually in depth, and drifts must be driven from the surface, to show the lateral extent of the deposit, its uniformity and richness. If
the rock mined carries from 1 to 2 per cent of fine copper, or from 20 to 40 pounds per ton, and the lode is 10 feet or more wide, the management knows that by mining on a large scale the costs can be brought down to a pay ing basis with copper at about 12 cents a pound. Some mines with wide deposits can make a handsome profit at the last named price. In the exploratory stages of a mine these factors must be ascertained before a permanent and economical plant can safely be installed. By neglecting this precaution several notable fail ures have been made and expensive plants erected only to be sold out later at a loss. In Michigan the copper deposits usually extend to great depths, some of the older mines being a mile deep vertically. Powerful hoisting engines and other machinery in proportion are ulti mately necessary for this work. The permanent shaft can be sunk in the footwall so as to en able the removal of the maximum of copper rock without endangering the shaft. The per centage of copper-rock that can be safely and cheaply mined often has an important bearing on the method of mining that is to be adopted. The shafts where possible follow the dip of the lode, thus avoiding the unnecessary mining of barren ground to reach the lode from the shaft, as would be the case if the latter were vertical. Shafts are now placed a thousand feet or more apart. The drifts from the shafts are driven along the lode from 85 to 150•feet apart, measured on the dip of the lode. From these shafts as starting points all of the lode matter possible is broken down from above by ustoying,° as far as the next higher level, by various methods; sorted where possible, and the barren portions left in the mine or returned to it from the surface, as °filling° to support the hanging wall. The amount thus discarded will vary according to the nature of the de posit, up to 40 per cent of the total rock mined. In some cases the copper is so evenly dissemi nated through the rock that sorting is im practicable and all of the rock goes to the mill. The copper rock from the scopes is trammed by hand, or where the shafts are far enough apart, by mechanical power, to the nearest shaft, hoisted in the cars in a cage, or in skips of a capacity of two to eight tons, dumped on the surface in the upper part of the rock house and here sometimes sorted again, or all of it delivered to breakers, by which it is crushed to small sizes; it is then transported to the stamp mill The larger masses of copper mixed with rock are placed under a heavy ham mer by which the rock is beaten off; the masses are then sent directly to the smelter. After going through the breakers the fine rock falls into large bins and is drawn off into railroad cars for transportation. To save expenses as many operations as possible are performed by gravity. A constant effort also is made to in troduce labor saving mechanical devices both underground and on the surface.
Stamping.— The crushed copper-rock from the mine in centre-dump railroad cars is taken to the stamp mill, which sooner or later forms part of the equipment of every permanent min ing company. Nature seldom furnishes a place that combines all the essentials of a good stamp mill site. The principal of these are an abun dant water supply, dumping-room for the waste products, sufficient elevation for handling the material by gravity through the different stages of the stamping process, accessibility by rail road and storage room for fuel. The absence of any one of these essentials adds materially to the cost of construction or to that of subse quent operation. Where possible, the side of a hill is selected near a body of water. A stream that will supply three and a half million gallons a day throughout the year is large enough for a one-head mill with a capacity to stamp from 500 to 700 tons of melaphyre or 300 tons of con glomerate in 24 hours. • If the stream can be dammed at an elevation sufficient to deliver its water by gravity at the mill about 40 feet above the point where the waste launders leave the latter, its water is utilized in that way. A dam built on a stream near the shore of Lake Su perior, to serve two mills that have a daily stamping capacity of about 4,000 tons, cost about $250,000. To pump water from Lake Superior for a mill of nearly the same capacity costs be tween one and one and a half cents per ton of rock stamped, to which must be added in terest and maintenance charges on a pumping plant that would cost nearly $100,000. To obtain water from Lake Superior requires a tunnel under the lake or a large pipe leading from the shore to a crib far enough out in the lake to avoid being choked by stamp-sand and other obstructions. This distance is usually about 1,000 feet.