Tufas and contemporaneous melted traps are very generally found together interstratified regularly with each other, and the inference to be drawn from their juxtaposition is of course simply this, that at one time liquid lava rolled along the bottom of the water, while at another showers of volcanic dust and cinders settled down in successive beds. This active play of the igneous forces took place at the mouths of estuaries or farther to sea; and it is accordingly sometimes not a little interesting to trace, amid the sediment that accumulated below the water during the pauses between the eruptions, well-preserved remains now of plants that had come drifting from the land, anon of slim spirifers, and producti that swarmed upon the hardened lava-streams, and amid the thickening volcanic mud that slowly sank to the sea-bottom. Such a sequence of events will be made plain from the following section, the materials of which are derived from different parts of the trappean region of Linlithgowshire. The undermost bed here shown (1) is one of marine limestone, abounding with encrinal joints, corals, spirifers, and other undoubtedly marine organisms. Above it comes a layer of tufa or volcanic ash (2) of a dull green aspect, the boundary line between the two rocks lying as clear as if the quarryman had marked it off with his foot-rule. The upper part of the ash, however, does not show an equally clear line of demarcation with the stratum above. On the contrary, it gradually changes its character, becomes more calcareous as it goes up, with here and there a stone-lily joint or a stray productus, until these organisms increase so much in number that the rock insensibly passes into an ordinary limestone (3) like that below. Next succeeds a thin seam of ash (4) resting sharply on the limestone and overlaid by a bed of shale (5) containing the same marine organisms. Another stratum of ash (6) resembling those below follows the shale, and is surmounted by a close compact greenstone (7) that hardens the ash on which it rests, but produces no apparent alteration on the soft fissile shale (8) above it. Next is a fourth seam of volcanic ash (9) resembling those below it, but without any shells or crinoidal joints, the only fossils observable being a few carbonized stems apparently of calamites and lepidodendra. Above it comes a bed of white quartzy sandstone (10) with similar vegetable remains, and then a layer of white stiff fire-clay (11) with rootlets of stigmaria, above which lies a seam of coal (12). A thin layer of soft blue shale (13) here intervenes, somewhat baked along its upper portions by another bed of compact vesicular greenstone (14), which displays in places a well-marked columnar structure. It is surmounted by a highly characteristic ash (15) in which there occur numerous large bombs chiefly of trap of different kinds, some of them highly vesicular. Fragments of shale also occur, mingled here and there