Page:Popular Science Monthly Volume 79.djvu/430

426

in some places there is a somewhat abrupt change from a bed of good coal of considerable thickness into coal of a much lower grade or into carbonaceous shale. In some places the movements have resulted in the coal being intimately mixed with the rocks of the roof and the floor. The roof and floor are most commonly of shale. One sometimes finds the roof to be of shale and the floor of sandstone, or vice versa; in a few places sandstone forms both the roof and the floor. The roof is frequently fractured to such an extent that in mining timbering will be necessary.

The coal of the region is of good quality, the best grade being anthracite, the poorest grade semibituminous. The average of 32 analyses of samples of coal taken by Dr. Martin so as to represent the coals of the whole field was as follows:

The fuel ratios of these coals varied from 3.61 to 15.88, the average being 7.78. The highest B.T.U. value was 15,574, the lowest 8,386 and the average of the 32 analyses was 13,174. The average analysis of 7 of the coals which were classed as anthracite was