strata, but becoming estuarine strata themselves. The thick limestones gradually thin out horizontally into a great series of sandstones and shales, with interbedded coal-seams, so that what bears evidence of a deep sea at the one end, gives proof of a muddy and sandy delta at the other. In other words, during the ages represented by what we call the Mountain Limestone, the central and south-western portions of England lay far below a wide breadth of ocean, and nourished a luxuriant crop of stone-lilies, mingled with the other denizens of the deep, while the Border district, and the whole of central Scotland, exhibited all the conditions of a vast delta, sometimes spreading out as broad verdant jungles, anon showing only scattered irregular groups of low, bare mud-banks and sand-spits, which at other times disappeared altogether beneath the dun discoloured waves. Now the reader will not fail to mark that this curious and interesting fact in the past history of our country, is ascertained solely from a comparison of fossil remains. The stone-lilies and shells of Derbyshire, and the lepidodendra and land-plants of the Lothians, form our sole basis of evidence, and we may rest on them with as perfect certainty as if they were so many duly attested documents deposited among the archives of our State-Paper Office. In our survey of the coal-field of Mid-Lothian, we have passed from the Lower Carboniferous estuary beds of Burdiehouse to the Middle Carboniferous marine beds of Roman Camp Hill, and their associated terrestrial strata,--the coal-seams and fire-clays. We come now, in our upward progress, to the Upper Carboniferous group, or Coal-measures proper.[65] These strata rest immediately upon the limestones, and attain a depth here of over three thousand feet. They consist of a great series of sandstones, shales, coals, and fire-clays, that vary in thickness from less than an inch to many feet, or even yards. The coal-seams are especially variable, many of them existing as mere films of carbonaceous matter; others varying up to a depth of fourteen feet. There are from fifty to sixty that exceed a foot, but the average thickness throughout the whole series is about three and a half feet.[66] They are nearly all underlaid by fire-clay or shale, and very generally have a roof of the latter material. [Footnote 65: These terms--Lower, Middle, and Upper Carboniferous, are used for want of others, and for the sake of clearness. They must not be regarded, however, as equivalent to similar groupings of the English carboniferous rocks, for the Scottish series is probably much older than the greater part of the English, and coeval, to a considerable extent, with the mountain limestone and millstone grit of the latter country.] [Footnote 66: See Milne on Mid-Lothian Coal-field. _Trans. Royal Soc. Edin._ vol. xiv. p. 256, whence the above details are taken.]