In some such way as this does the Roslyn sandstone series appear to have originated. I have indicated what seems to have been the main features in the process, but it was probably a very complex one. There may have been a great many oscillations of level of variable effects, some of them raising the disturbed area to a much greater height at one point than at another. This inequality would of course produce marked effects along a low flat country such as that at the mouth of a great river. New currents would be produced and the direction of old ones changed; great shoals and banks of silt would be worn down only to be thrown up again at some new point, where another oscillatory movement would expose them afresh to destructive denudation. The variations in the amount of elevation and depression would likewise modify the transport of detritus to the sea, and give rise to a varied and ever-changing sea-bottom. In short, the alternations and variations must have been endless, for to the ordinary multiplied interchanges of a delta we must add those induced by a constant and unequal oscillation of the earth's crust. The Roslyn sandstone series comes to a close, and passing onward in ascending scale we meet with another great group of coal-bearing strata. They occupy the central area of the Mid-Lothian coal-field, and from their gentle inclination as compared with the lower strata that rise up from under them on either side of the basin, are known as the _Flat Coals_. Their total thickness--that is to say, all that has escaped denudation--amounts to a thousand feet or more. They consist chiefly of sandstones, shales, ironstones, and fire-clays, with from twenty to twenty-five seams of coal, of which sixteen are thick enough to be worked. Their similarity to the Edge coals below points to a similarity in the conditions of formation. The frequent alternations of sandstone and shale show how the delta gradually pushed outwards again and re-occupied its ancient site above the successive forests of the Edge series and the successive coral-beds of the Roslyn group. The coal-seams indicate the further progress of the detrital accumulations, and the eventual formation of vast swampy flats that nourished a rank growth of stigmariæ, and tracts of drier ground waving with ferns, and shadowed by the spiky foliage of the club-moss and the broader fronds of the tree-fern.