Page:Popular Science Monthly Volume 79.djvu/429

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of the formation are best exposed, more than a dozen seams of coal may be seen, but several of these are thin and unimportant. In places, the best coal beds have thicknesses of more than 25 feet of good coal; at many places beds exceeding ten feet may be seen. Owing to the complexity of structure and the small amount of development work it is impossible to correlate the coal beds in one part of the field with those of other parts, even when the outcrops are not widely separated. The thicknesses of the beds often vary greatly within short distances along both the strike and the dip. In some places the evidence suggests that the irregularities in thickness are due to movements, the coal having been squeezed into great pockets of irregular shape. Moreover,