Rather more than 300 feet above the limestone of Burdiehouse there occurs in the Mid-Lothian coal-field a series of shales and seams of limestone. The former are sometimes black and hard, sometimes bluish-grey, soft, and frequently imbedding the remains of several genera of mollusca and other organic remains. The limestones vary considerably in the thickness and general aspect of their several seams, some being highly crystallized and about two or three feet in depth, others dull, compact, and ranging up to twenty and thirty feet thick. The shales and limestones are intercalated with and sometimes pass into each other, through the gradations of shaly limestone and calcareous shales. The whole series may measure 150 to 200 feet, resting on the Straiton sandstone below, and passing upwards into the under part of the coal-bearing strata of Mid-Lothian known as the _Edge series_. These limestones form the northern _marine_ equivalents of the mountain limestone of England, while the sandstones and shales on which they rest, including the Burdiehouse beds and all the Lower Carboniferous group, must probably be regarded as _estuarine_ equivalents of the same formation. That is to say, while marine limestones were accumulating over the site of central England, sandstone, shale, and drifted plants, were slowly gathering in a wide estuary over what is now central Scotland, and only at the close of the period did marine limestones form simultaneously at both localities. In examining these Mid-Lothian beds we are struck at once with the great dissimilarity that obtains between their organic remains and those of the underlying strata. All the land-plants disappear--ferns, lepidodendra, sigillariæ, and stigmariæ. The cyprides, too, no longer occur, though the shales seem, at a first glance, to differ in no respect from those underneath, in some of which the cypris-cases were seen to abound. Neither can we detect the glittering scales and teeth that stood out in such strong relief upon the rocks below. Yet the fossils are scarcely less numerous than they were in the lower beds. Nay, in some of the limestones they lie so crowded together that the rock seems entirely made up of them. Plainly such a total renovation of organic life points to some equally extensive change of a physical kind. Let us examine for a little some of the fossil remains occurring in the mountain limestone series of Mid-Lothian, and read off, if we can, the revolutions which they chronicle. [Illustration: Fig. 32.--Section from Gilmerton to Crichton; _a_, Lower Carboniferous; _b_, Mountain Limestone; _c_, Edge Series; _d_, Roslyn Sandstone Group; _e_, Flat Coals; _y_, Drift.]