The southern part of Mid-Lothian consists of a broad heathy moorland, that slopes northward into the more cultivated country, and swells upward to the south into the series of undulating ridges that form the Moorfoot Hills. It is traversed by several streams which rise high among the pasture grounds of the south, and flow some into the valley of the Esk, and thence into the sea at Musselburgh; others past the ancient fortalices of Borthwick and Crichton, and so by the valley of the Tyne into the sea at Tyningham. In their upper course they traverse a broad belt of the mountain limestone that stretches across this part of the country from east to west, and dips away north under the coal-field. Where the streams have been able to cut through the thick mantle of heath, sand, gravel, and clay, by which these higher grounds are covered, we sometimes obtain a complete section of the strata displayed in regular sequence along the bottom of the channels. Thus, one of the rivulets that trickles slowly through the swampy ground of Middleton Muir, on approaching the line of limestone begins to descend more rapidly, and has excavated its course through several feet of the rock below. The limestones are well exposed along each side of the stream, forming in some places steep walls tapestried with moss and overhung with scraggy furze, and offering to the student an instructive series of sections. Near the farm of Esperston, where the stream flows through a narrow secluded valley, the limestones form a floor which the water in the course of centuries has worn smooth, so that the rock with its included encrinal stems and shells, polished by the ceaseless flow of the current, shows like a sheet of variegated marble. At one point on the side of the water-course the observer may notice a low ledge of rock jutting out for a short way along the edge of the stream. The upper part is a hard compact limestone, full of small crinoidal joints. The bed underneath it has been greatly eroded by the rivulet, but enough remains to show that the stratum is one of coal. It rests upon the series of limestones and sandstones seen in the upper part of the water-course, and is surmounted by the thick limestones of Arniston and Middleton. A similar seam nineteen inches thick has been worked among the limestone about three miles to the west at Fountain. The same bed occurs among the quarries on Roman Camp Hill already mentioned, and I have seen an equivalent stratum intercalated among sheets of cup-corals and stone-lilies on the shore at Aberlady, where the waves have laid open perhaps the finest section of Carboniferous limestone strata in Scotland. In West-Lothian, too, the same intercalation of coal-seams among the mountain limestone beds can be seen in many places. Thus, in the bed of the River Almond, near Blackburn, the following section is laid bare:--