Abstract: A method and apparatus for mining a desired geological strata, such as a coal seam, from surrounding undesired geological strata. A bore is formed in the coal seam in a first direction, and while progressively penetrating the coal seam with a mining machine, the distance of the mining machine from the undesired strata in a second direction perpendicular to the first direction is effectively measured. The measuring is accomplished by generating chips by boring upwardly through the coal seam toward and into the undesired strata, and reflecting light off of the chips, the difference in reflectivity between the chips from the coal and from the undesired stratum serving to indicate when the undesired stratum has been reached. The distance of upward movement of the drill from a reference until the reflectivity of the undesired stratum is detected is measured, or the thickness of the coal seam above the machine is measured, and the mining machine is controlled based upon such measurements.