But this is not all. We must attend to the character of the organisms as well as to their order of occurrence. We must distinguish the animal from the vegetable, the terrestrial from the marine, and scrupulously examine the peculiarities of each so as to recognise them again in other strata. By such careful scrutiny we may trace out the successive changes in the physical aspect of a district during past times, viewing in terrestrial plants (when clearly occupying their original site) evidence of an old land-surface; in _cyprides_, _unios_, and _paludinæ_, traces of a former lake; and in corals and marine shells, unmistakable proofs of an ancient sea-bottom. Still further, by marking the specific character of such fossils we obtain a key to the age of many rocks that otherwise would be unintelligible, for it is found that each of the stratified formations, from the oldest upwards, has its own peculiar and characteristic organisms recognisable all over the world. This test of the geological position and age of any fossiliferous rock has a peculiar value, for it can be applied with infallible success where every other fails. The order of superposition is often obscured by dislocations and other causes, and the mineralogical texture of a formation may change entirely in a short space; but if the imbedded fossils remain, we can be at no loss as to the relationship of the rock which contains them. And hence, if in some lone island of the Hebrides, haunted only by the screaming sea-fowl, we find a patch of shale containing ammonites, belemnites, and a host of other shells in large measure identical with those occurring among the clays and limestones of Gloucestershire, we infer that they must all belong to one series and be of the same age; that, as we know the English beds to form part of a formation called Lias, of which, the exact place in the geological scale has been ascertained, so in like manner the Scottish beds must occupy a position in the same series; and that consequently there was a time when the site of Cheltenham and part of the Hebrides lay each beneath a sea which teemed with ammonites, belemnites, and many other mollusca, along, too, with the bulky saurians of the Lias. And yet no study of the surrounding rocks in the northern locality, even if carried on for a thousand years, could ever have thrown one ray of light upon the subject. In an earlier page our grey rounded boulder was introduced to the reader as a mass of sandstone belonging to the Carboniferous group of rocks. How could one be sure of the precise geological age of a loose water-worn block that might have journeyed all round the world? Simply by its included fossils. The calamite, lepidodendron, and stigmaria, revealed the date of the stone as clearly and unmistakably as if we had seen it lifted from its original bed by the lever and crane of the quarryman. These plants are peculiarly characteristic of the Carboniferous strata, and they consequently