MINING CONDITIONS UNDER CITY OF SCRANTON, PA.
52
find that when they thus remove all the coal, a gradual though small surface settlement results, the amount of which depends upon the depth and the thickness of the coal. In this country, however, we can not hope to profitably use such expensive mining methods as may obtain abroad, because the cost of labor in the United States is very much greater and the market price of coal very much less than in Europe. The appearance of culm flushing is shown by Plate 26.
COGS.
It is frequently necessary in the course of mining to make use of some roof-supporting device that may be quickly constructed and is withal possessed of great strength. The timber crib filled with mine rock—known in mining parlance as a “cog”—has been found to answer these conditions in a very satisfactory manner, and is extensively used in all coal-mining districts. The cog consists simply of a rough crib of stout logs placed one above the other, log-house fashion, the spaces between the logs being chinked and the interior being filled with rock from the mine. (See PI. 27 A.) This construction is quickly erected, and possesses great strength. Of course it is not permanent because the timbers decay in a few years. Cogs are mainly used for the purpose of stopping a settlement, squeeze or creep which the mine foreman !mows to be imminent or in progress. A view of a squeezed area in the Dunmore bed is shown in Plate 29 A. When sufficient cogs are placed in proper localities the strata above the bed will frequently crack through to the surface, and the progress of the squeeze or creep will thus be stopped.
GOB PIERS.
Gob piers are pillars built of such refuse rock as may be readily found in most mines—mainly bony coal, fire clay, and slate. Such rock is mostly soft and does not possess very great compressive strength. Some of these piers have a square outer or inclosing wall, and are filled with mine refuse shoveled into them. In others the interior is laid up by hand and the rocks are more carefully compacted by filling the voids with fine mine refuse.
A great many such piers have been built under localities in the city of Scranton for the purpose of supporting the roof under valuable surface improvements. (See Pis. 27 B and 28.) The value of such piers, that is, their supporting strength, depends upon the compressive strength of the materials of which they are constructed. The value will be greater if the voids between the larger pieces are filled with the small rock and shoveled material from the mine.
GOB STOWAGE IN ROOMS.
Most coal beds consist of interstratified layers of coal, fire clay, slate, and bony coal; the three latter, of course, compose the prin-