[Footnote 72: Some observers have pointed to the occurrence of vertical and inclined trunks of trees in the Carboniferous sandstones, and deduced therefrom what has seemed to them a triumphant argument in favour of the rapidity wherewith our coal-fields must have formed. A foundered tree, they say, sank with its heavy-laden roots among the sand at the bottom, its stem pointing up into the water like the snags of the Mississippi, so that the sand must have come rapidly down to entomb the whole before it had time to decay, and thus thirty or forty feet of sediment must have been deposited in a few years, perhaps even months. But this is somewhat like a begging of the question. We have yet to learn how long a water-logged trunk will resist decomposition.] The Edge coals of the Mid-Lothian coal-field are succeeded by a group of sandstones and thin shales, with three or more seams of limestone. This group of strata, which we may call the Roslyn Sandstone Series, reaches a thickness of from 1200 to 1500 feet, and serves as a middle zone to divide the Edge coals below from the Flat coals above. It contains only a few thin laminations of coal, and these chiefly at its upper and under portions. Such a great intercalation of beds, without coal-seams, points, we might readily conjecture, to some change in the physical conditions of the ancient delta. The nature of this change can be easily made out from an examination of the rocks, and the reader will see that here again we are indebted to fossil remains for the most conclusive and satisfactory evidence of these old physical revolutions. The absence of coal-seams suffices to indicate that during the formation of the middle group that part of the delta occupying the site of Mid-Lothian was continually submerged, and never rose to the surface so as to allow a covering of vegetation to form upon it.[73] The large beds of sandstone prove a continued transport and deposition of detritus during undisturbed periods of considerable length. The intercalations of shale, pointing to local changes in the currents or other modifying causes, are usually of small thickness and extent, while the sandstone beds sometimes attain a depth of 150 or 200 feet, and extend over wide areas of country. So far these mechanical rocks indicate the deposition of sand and mud under water, but whether at river-mouth or sea-bottom is left uncertain. From the fossil remains, however, we learn that the deposition took place in the sea, but at no great distance from land; in other words, the area of Mid-Lothian, which, during the accumulation of the edge coals, had been alternately clothed with vegetation and inundated by the river, sank down many fathoms, so that the sea rolled over it and all its submerged forests. The proof is two-fold, first, from the character of the organic remains in the limestones; and second, from that of those in the sandstones and shales.