[Footnote 64: If it be correct to set down the North Greens coal-seam as really representing a terrestrial surface, that is, of course, a flat delta or plain scarcely raised above the sea-level, we must, I suspect, call in the aid of a slight elevatory process, or else hold that the depth of the sea at the locality where the lower limestone was forming did not exceed 80 or 90 feet, and may have been considerably less, and that this space came to be eventually filled up by the detritus of the river. But the wide extent and sometimes the great thickness of the limestone beds seem to indicate a greater depth, and thus favour the idea of an elevation of the sea-bottom to form the North Greens coal-seam.] Now this condition of things differs entirely from what is presented by the Mountain Limestone group of England. That formation, when typically developed, attains a thickness of from 1000 to 2000 feet, and gives rise to that green hilly kind of scenery whence it has derived its name. It is unequivocally a marine deposit, since it abounds in corals, echinoderms, brachiopodous molluscs, and other productions of the deep. Northward, however, it undergoes a gradual change, getting greatly thinner, and split up by a series of intercalations of shale and sandstone. This alteration goes on until, on the border-land between the two countries, the massive limestone of Derbyshire has dwindled down into a series of thin beds, often widely separated by intervening strata, which contain many seams of coal. After crossing the Silurian district, and descending the northern slopes of the Lammermuir Hills, we get into the Carboniferous system again, and find its limestone series still farther diminished. With this decrease of marine formations, we can detect an augmentation of coal-bearing strata. Thus the Berwickshire coal-field lies in this lower set of beds, far under the coal-measures of Newcastle. In the Lothians, too, as has been shown, coal is extensively worked in the same series, and these seams also find their representatives in Fife and Lanarkshire. The gradual change from the kind of strata found on the horizon of the Burdiehouse limestone, to those occurring on the horizon of the Mountain limestone, indicates, as we saw, a gradual change of the conditions of deposition; and the nature of this alteration is shown by the difference in the character of the imbedded fossils. The passage of the massive Derbyshire limestone into the thin limestones and coal-bearing sandstones of the north, as decidedly marks another change in the relative position of sea and land. The former was a succession in time, the latter was one in space, but the mode of reasoning remains the same for both. In the former case, we saw estuarine strata passing upward into others wholly marine, and the order of superposition told us that the locality was first an estuary, and then slowly became open sea. In the latter case, we see marine beds not succeeded by estuarine