Every one knows that basalt, lava, pumice, scoriæ, and ashes, are the various matters ejected from volcanoes. When these materials are found interstratified among the various geological formations, they are termed _trap_-rocks,--a name derived from the Swedish _trappa_, a stair, in allusion to the step-like or terraced appearance which they often present. They are of all ages, having been detected in the lower Silurians of Wales, and in the deposits of all subsequent periods up to the volcanic eruptions of the present day; thus evidencing, that from the remotest times there have been Ætnas and Vesuvii slumbering perhaps for ages, and then awakening to lay the surrounding districts in ruins. I have already said that the rocks from which the geologist has to compile his history, are mostly relics of the sea; hence most of the trap-rocks which he meets with in his explorations are the products of submarine eruptions. Far away down among the Silurian rocks, he can trace the floor of a primeval ocean thickly covered with stone-lilies, trilobites, and molluscs, and in following it out he marks how ashes and lapilli, ejected from some submarine orifice, settled down amid the organisms and well-nigh destroyed them, while at other times streams of molten matter were poured out along the sea-bottom, and hardened into masses of solid rock. He sometimes even encounters what seems the vent whence these eruptions proceeded, filled up now by a boss or plug of hardened trap, but he never can detect any trace of land. Some of these oceanic volcanoes may, like Graham's Island in the Mediterranean, have raised their tops above water, sending clouds of steam and cinders far and wide through the air, but the waves would eventually wear down the new-born land, and scatter its broken fragments along the floor of the sea. Among the carboniferous rocks of Scotland, however, we meet with a different condition of things. There, too, we can trace out submarine lava-streams, and mark how showers of ashes destroyed the delicate organisms of the deep; but we encounter, besides, undoubted traces of a land, not parched and ruinous as though the igneous forces had laid it waste for ever, but thickly clothed with vegetation of a more luxuriant type than that which clusters over Vesuvius and Calabria, or lies spread out across the "level plains of fruit-teeming Sicily."[74] We have looked at the plants and animals of the Carboniferous era; its rivers and deltas; its slow elevations and depressions of the ground. It may, perhaps, complete the picture of that ancient period, if we examine, though but briefly, its igneous eruptions, the more especially since these may be regarded as, to a considerable extent, typical of trap-rocks belonging to every age and every country. [Footnote 74: Της καλλικἁρπον Σικελἱας λενροὑς γὑας. Æsch. _Prom. Vinct._ 369--a passage graphically descriptive of an ancient eruption of Ætna.]