We have seen above that the area of a delta is often partially submerged below the sea, and that such changes may become of the most marked kind where the country is liable to be depressed by earthquakes. There can accordingly be no difficulty in understanding how the ancient carboniferous delta of Mid-Lothian may have likewise subsided. But the limestones are unmistakable evidence that not only was the area of the delta submerged, but that for a while no sediment was deposited over it, and hence marine animals peculiar to clear water flourished so long and so abundantly as to form by their remains several beds of limestone. Had these beds been merely local we might have regarded them as having been deposited in lagoon-like portions of the delta, shut out from the detrital matter of the river on the one side and open to the sea on the other. But their wide extent and nearly uniform thickness preclude such a supposition. The following explanation appears to me the most probable:-- After the series of the Edge coals had been brought to a close, the coal-fields of Scotland underwent a complete submergence below the sea. This depression was probably very gradual, yet more rapid than that long-continued one which had been going on during the earlier part of the Carboniferous series, and the consequence of this greater rapidity was to prevent the growth of stigmaria swamps or reedy jungles, by keeping the alluvial surface continually sunk to some depth below the water. The amount of subsidence until the deposition of the lowest limestone may not have been great, but even a slight depression would tell vastly on an area of flat delta land. Mud banks would be brought down into the region of waves and surface-currents, and speedily be spread out over the floor of the sea. Forest-covered islands would in like manner be levelled down, and their trees sent drifting seaward or submerged amid the re-formed silt. Thus altered, the delta would sink below the sea, and the sediment borne down by the river would be scattered out over the older deposits as a slowly-forming sheet. By degrees this detrital matter must have been carried less and less farther out to sea; in other words, the area of deposit or delta must have crept gradually nearer to the land--a result owing partly to the recession of the ancient coast-line, and partly perhaps to a greater amount of depression inland than at the coast, which would of course lessen the velocity of the streams and cause them to deposit their burden of sediment at higher levels than before. The consequence of this retreat of the delta from the sea would be to purify the water over the site of the old swamps, and render it fitted for the habitation of corals, molluscs, and other marine animals. A medium thus prepared would not be allowed to remain long untenanted, and so we find that it came to be densely peopled with the organisms peculiar to such a station. Stone-lilies, cup-corals, net-like bryozoa, molluscs of many