for local supply. Of the central area three-fourths are in Illinois, less than one-sixth in Indiana, and about one-twelfth in western Kentucky. In the western field the most extensive mining operations have been carried on in Iowa and Missouri; its area is greater than that of any other one Coal field in the United States. The coals are of great variety; the best which has so far been mined is that of the Indian Territory. The Rocky Mountain Coal beds have have been found in the geological formations from the Carboniferous up to and including the Cretaceous, differing in this respect from those hitherto enumerated, which, with the exception of that in Virginia and North Carolina, are all confined to the Carboniferous. Coal has been mined in the Pacific States.
Coal Mining.—The cutting of a path through the harder rocks, as carried on by the ancient miners, was particularly laborious and unhealthy. Miners became subject to disorders of the lungs at an early age. Previous to the introduction of blasting, the implements used were wedges and hammers. Bit by bit pieces of rock were broken away, the operation being assisted by natural fissures in the rock and by the brittleness of the hard material. In this way the ancient miners cut coffin-shaped galleries 5 feet in height. At the present time the galleries or levels are usually 7½ feet high and 5 feet wide, thus affording facilities for traveling and for ventilation. Gunpowder was not applied to mining purposes until the beginning of the 17th century, and it made its way so slowly that it was not largely employed until the 18th century. Of late years rock-drills driven by steam or by compressed air have come largely into use. The bore-hole, when finished, is then charged. The gunpowder is inclosed in a little bag of cloth dipped in pitch and provided with a fuse. The fullest benefit of modern explosives, such as dynamite, gun-cotton and yonite, can be obtained only by the use of strong detonators fired by electricity, by which it is impossible to place a number of bore-holes in such a manner that when fired simultaneously they shall help one another. Blasting powder is still used for removing coal and millions of tons are obtained by its aid. In order to obviate the danger of explosions in fiery collieries, many ingenious substitutes for blasting have been proposed. For example, a hole is bored and wedges inserted to force down the Coal which has previously been under-cut with the pick.
JEFFREY LOADING CONVEYOR USED IN A COAL MINE
Various machines have been invented with a view of lessening the labor and expense of under-cutting coal seams. They work with compressed air or electricity, and have the cutters arranged on the periphery of a rotating disc, or on a traveling pitch chain. The coal, when broken down, is placed in cars and drawn to the bottom of the shaft and raised to the surface. The actual mode of working the coal varies greatly in every district. By the post-and-stall, or board-and-pillar, or (in Scotland) stoop-and-room, method the first stage of excavation is accomplished with the roof sustained by coal; in the long-wall method the whole of the coal is allowed
