The origin of the constant succession of coal seams, sandstones, and shales, of the Edge series may be thus accounted for. The area of Mid-Lothian formed part of a great delta, which, like that of the Ganges, was undergoing a gradual subsidence during the Carboniferous era. The rate of this movement probably varied at different times, and might even be occasionally interrupted by short periods of elevation. When the ever-increasing accumulations of silt brought down by the river reached or nearly reached the surface of the water, they would become the site of wide tracts of swampy vegetation that flourished for hundreds or thousands of years. Eventually, however, these jungles, invaded by the changing currents of the river, were buried beneath a thick deposit of fluviatile sediment, or more probably the vegetation might become unable to keep pace with an accelerated rate of submergence, and the forests would then be tranquilly carried down beneath the water, and soon covered over with sand and mud. The detrital matter might in like manner continue to be deposited over the sunk forest for many years, perhaps centuries, until the muddy bottom again reached the surface, and once more waved green with sigillariæ, calamites, and lepidodendra. Another long interval might here elapse, in which a thick bed of vegetable matter might accumulate, much after the manner of the formation of peat among the bogs and mosses of our own country. The periodical inundations of the river probably gave rise to wide marshes and lagoons, often tenanted by lacustrine shells, and thickly overgrown with aquatic vegetation. The decaying plants decomposed the red ochreous matter with which the water was charged, and re-deposited it among the mud and rotting leaves at the bottom as a carbonate of iron. Such ferruginous accumulations, often entombing fern-stems and other plants, with scales and teeth of ganoidal fishes, sometimes _conulariæ_ and _lingulæ_, and, in certain localities, whole acres and miles of fresh-water shells, are known now as our _clay-band_ and _black-band ironstones_. We can easily conceive that, in shallower parts of the lagoons, a dense growth of marshy plants might spring up, preventing any deposition of iron, and when the whole came to be covered over with later accumulations of sand or mud, the deeper parts of the old lake would be covered with a seam of ironstone, and the shallower portions would display a bed of coal. In some such way we may account for the frequent passage of ironstone into coal, and coal into ironstone in many of our coal-fields. If undisturbed by the ever changing currents of the river, these wide expanses of marsh and lake might continue for many long years, the constant evaporation being counterbalanced by continual supplies of water from the main stream. Eventually, however, owing perhaps to another period of more rapid submergence, the water gained the ascendency, and once more rolled over