The Flat coals are not succeeded by any other palæozoic strata. Above them stretches the drift already described: sometimes in the form of a stiff blue clay resting on a striated rock-surface; sometimes as a coarse gravel containing fragments of all the rocks in the neighbourhood; and sometimes as a fine white sand diagonally laminated, and often showing dark partings of coal-fragments. From the section given above (Fig. 32) at p. 196, the reader, will see that as the upper limit of the Flat coals is formed by the drift, a large part of that series may have been borne away by denuding agencies. Had there been even a seam of limestone above them, it would have sufficed to show their true thickness, for we should then have seen, that how much soever had been removed in later times from above the limestone, nothing had been removed from below it; and so it would mark the true original limit of the series. We cannot now tell how much thicker the upper part of the Mid-Lothian carboniferous system may have been. Probably, during the long ages that intervened between palæozoic and post-tertiary times, many hundred feet were borne away and carried to other sites, there to grow up into new islands and continents, clothed with other types of verdure, and peopled by other races of animals, and fitted to become, in a long subsequent period, the dwelling-place of man. In fine, the evidence of these ancient changes in the history of the Mid-Lothian coal-field is derived, as we have seen, from two sets of facts; first, those of a mechanical, and, second, those of an organic kind--the one class explaining and confirming the other. Beginning our investigation at the horizon of the Burdiehouse limestone, we saw the curtain rise slowly from off a wide estuary, in which there gambolled large bone-covered fishes, while huge pine-trees--branchless and bare, seed-cones, fern-fronds, and twigs of club-moss, floated slowly away out to sea. The panorama moved on, and brought before us the ocean-bed of the Roman Camp limestone, with its groves of stone-lilies and bunches of coral; its tiny shells moored to the bottom, or creeping slowly athwart the limy floor, or spreading out their many arms, and rising or sinking at will. This picture passed slowly away, and then came the delta of the Edge coals, with its sand-banks and ever-shifting currents, its stigmaria swamps, and its forest-covered islets. We saw the delta gradually sink beneath the sea, and corals and stone-lilies cluster thick over its submerged area, to form the limestones of the Roslyn group. Again, the mud-bars of the river crept out to sea, and tangled forests waved green as of old, washed by the sea or inundated by the river. How this last period came to a close, we shall probably never know, and have no possible means of conjecturing. We pass at one step from the ancient era of the coal to the comparatively modern one of the drift--from a verdant palæozoic land, to an icy post-tertiary sea.