Copper ores are found in various conditions, but there are no mines in the world where there is so much of it in the native state as in the Lake Superior region. The largest masses stand upon their edges in the vein. Sometimes they are many yards in length and several feet in breadth, and their thickness varies from an inch to more than a yard. Silver is associated with copper, sometimes in occasional lumps unattached to the copper, though generally the two metals are in contact, as if the silver had been deposited with the copper, without forming an alloy with it. I believe no alloy of the two metals is ever found there, and consequently they can never have been in a fused condition in contact. The lumps of silver vary in weight from a few grains to several pounds.
Masses of copper of great size have been found. One was discovered several years ago containing at least five hundred tons, and other masses are said to have contained more than eight hundred tons.
BLASTING COPPER MASSES.
The work of cutting out one of these masses sometimes occupies several weeks or months. The mass fills the entire vein, so that the rock must be removed on one side; the mass is thus left as a wall, its upper edge extending into the roof and its lower edge into the floor. When the side is laid bare, the mass is attacked at one end by introducing charges of powder, and as fast as room is obtained, and cracks are opened between the copper and the rock, the size of these charges is increased. The usual form of blasting under such circumstances is by what is known as the “sand blast.”
Powder is poured loosely into the openings in large quantities, and is then covered with dry sand. It is lighted by means of a safety fuse, which gives the men time to escape. The first blast will be in a small cavity, and as the cavities are enlarged more powder is introduced, until sometimes several hundred pounds are spread in the crevice and fired at once. In several instances one thousand pounds of powder have been burned at a single blast. When the mass has been thrown down in this way, the work of cutting it so that it can be removed begins.
Two miners strike in turn upon a long steel chisel held by a third, and thus gradually cut a groove across the copper. This work is repeated until the mass is cut through, and sometimes it requires an entire month to make a single cut. The mass is reduced into pieces weighing five or six tons each, which can be hoisted up the shaft, and it will then be cut into little morsels weighing not more than two or three tons each, so that it can be easily handled.
To give an idea of the extent of this kind of work, it may be stated that some years ago a mass of copper was uncovered on which nearly three thousand pounds of powder were expended before it was thrown over so that it could be cut. When it was thrown over it was forty-five feet long, and its greatest thickness was eight feet. Several months were required for cutting it up so that it could be removed, and it was estimated from its measurement to weigh more than five hundred tons. This mass copper usually yields more than ninety per cent. of pure metal.
INTERIOR OF A COPPER MINE.