Description of the Region. Geographic Position. —The region discussed in this paper comprises a thousand square miles around Harper’s Ferry, about equally divided between Virginia, West Virginia and Maryland. Topography and general Geology .—The topographic features of the region are, briefly, two mountain lines and three valleys. In order, from west to east, they are Shenandoah valley, Blue ridge, Middletown valley, Catoctin mountain, and Leesburg valley. The position of the ridges is shown on the geologic map by the black areas, the sandstones and the ridges being nearly everywhere coincident. The last valley is composed of Mesozoic rocks and does not concern the question under discussion; the middle or Middletown valley is composed of schists, injected with granite dikes, and will only be alluded to briefly; the first or Shenandoah valley is occupied by the Potomac and Shenandoah rivers, and is almost entirely floored by the lower Silurian limestone. Of the . . .