Whether or not the same type of fossils was always contemporaneous over the whole planet cannot be determined; but it generally occupied the same place in the procession of life. Hence stratified formations, which may be quite unlike each other in regard to the nature of their component materials, if they contain similar organic remains, may be compared with each other, and classed under the same name. Fossils characteristic of particular subdivisions of the series of geological formations are known as type-fossils, of which the following are examples : Lepidodendra and Sigillariæ, characteristic of Old Red Sandstone and Carboniferous rocks (pp. 259, 272, 273). Cycads, characteristic of Mesozoic rocks (pp. 293, 299, 314). 253, 264, 277). Cambrian to Carboniferous rocks (pp. 244, Cystideans, characteristic of Silurian rocks (Fig. 123). Carboniferous rocks (Fig. 150). Blastoids Ichthyosaurus and Plesiosaurus-Mesozoic rocks (Fig. 180). Toothed birds-Cretaceous rocks (p. 321). Nummulites, Palæotherium, Anoplotherium, Deinocerata, characteristic of older Tertiary rocks (pp. 327-338). Mastodon, Elephas, Equus, Cervus, Hyæna, Apes, characteristic of younger Tertiary and Recent rocks (pp. 339-365). By attentive study and comparison, the fossiliferous rocks in different countries have been subdivided into sections, each characterised by its own facies or type of organic remains. Consequently, beginning with the oldest and proceeding upward to the youngest, we advance through natural chronicles of the successive tribes of plants and animals which have lived on the earth's surface. These chronicles, consisting of sandstones, shales, limestones, and the other kinds of stratified deposits, form what is called the Geological Record. In order to establish their true sequence in time, their Order of Superposition must first be determined; that is, it is requisite to know which lie at the bottom, and must have been formed first, and in what order the others succeed them. When this fundamental question has once been settled, then the fossils characteristic of each group of strata serve as a guide for recognising that group wherever it may be found. While fossils enable us to divide the Geological Record into chapters, they also show how strikingly imperfect this record is as a history of the plants and animals that have lived on the surface of the earth, and of the revolutions which that surface has undergone. We may be sure that the progress of life, from its earliest appearance in lowly forms of plant or animal, has been continuous up to the present condition of things. But in the Geological Record there occur numerous gaps. The fossils of one group of rocks are succeeded by a more or less completely different series in the next group. At one time it was supposed that such breaks in the continuity of the record marked terrestrial convulsions which caused the destruction of the plants and animals of the time, and were followed by the creation of new tribes of living things. But evidence has every year been augmenting to indicate that no such general destruction and fresh creation ever took place. The gaps in the record mark no real interruption of the life of the globe. They are rather to be looked upon as chapters that have been torn out of the annals, or which never were written. We have already learnt in Chapter VIII how many chances there must be against the preservation of anything like a complete record of the life of the globe at any particular time. It is also clear that even where the chronicle may have been comparatively full, it is exposed to many dangers afterwards. The rocks containing it may be hidden beneath the sea, or raised up into land and entirely worn away, or entombed beneath volcanic ejections, or so crushed and crumpled as to become no longer legible. Taking fossils as a guide, geologists have partitioned the fossiliferous rocks into what are called stratigraphical subdivisions as follows:-A bed, or limited number of beds, in which one or more distinctive species of fossils occur, is called a zone or horizon, and may be named after its most typical fossil. Thus in the Lias, the zone in which the ammonite known as Ammonites Jamesoni occurs, is spoken of as the "zone of Ammonites Jamesoni," or "Jamesoni-zone." Two or more zones, united by the occurrence in them of a number of the same characteristic species or genera, form what are known as Beds or an Assise. Two or more of such beds or assises may be termed a Group or Stage. Where the number of assises in a stage is large they may be subdivided into Sub-stages or Sub-groups. The stage or group will then consist of several sub-stages, and each sub-stage or subgroup of several assises. A number of groups or stages is combined into a Series, Section, or Formation, and a number of series, sections, or formations constitute a System. A number of systems are connected together to form each of the great divisions of the Geological Record. This classification will be best understood if placed in tabular form, as in the subjoined subdivisions, which occur in the Cretaceous System.1 Two or stages Beds or an assise Rotho Warminster beds. more groups or Series, section, or for- Neocomian formation. Several related forma- } = System tions Cretaceous System. The names by which the larger subdivision of the Geological Record are known have been adopted at various times and on no regular system. Some of them are purely lithological; that is, they refer to the mere mineral nature of the strata, apart altogether from their fossils, such as Coal-measures, Chalk, Greensand, Oolite. These names belong to the early years of the progress of geology, before the nature and value of organic remains had been definitely realised. Other epithets have been suggested by localities where the strata occur, as London Clay, Oxford Clay, Mountain Limestone. The more recent names for the larger divisions have, in general, been chosen from districts where the strata are typically developed, or where they were first critically studied, e.g. Silurian, Devonian, Permian, Jurassic. In some cases, the larger subdivisions have received names from some distinguishing feature in their fossil contents, as Eocene, Miocene, Pliocene.2 But it is mainly to the minor sections that the characters of the fossil contents have supplied names. The designation of any particular group of strata has gradually 1 For an account of the Cretaceous System, see Chapter XXIV. come to acquire a chronological meaning. Thus we speak of the Oolites or Oolitic formations of England, and include under these terms a thick series of limestones, clays, sandstones, and other strata, replete with organic remains, and containing the records of a long interval of geological time. But we also speak of the Oolitic period—a phrase which, in the strict grammatical use of the word, is of course incorrect, but which conveniently designates the period of geological time during which the great series of Oolites was deposited, and when the abundant life of which they contain the remains flourished on the surface of the earth. This chronological meaning has indeed come to be the more usual sense in which the names of the major subdivisions of the Geological Record are generally employed. Such adjectives as Devonian and Jurassic do not so much suggest to the mind of the geologist Devonshire and the Jura Mountains, from which they were taken, nor even the rocks to which they are applied, as the great sections of the earth's history of which these rocks contain the memorials. He compares the. Jurassic or Devonian rocks of one country with those of another, studies the organic remains contained in them, and then obtains materials for forming some conception of what were the conditions of geography and climate, and what was the general character of the vegetable and animal life of the globe, during the periods which he classes as Jurassic and Devonian. Summary.-Fossils are the remains or traces of plants and animals which have been imbedded in the rocks of the earth's crust. From the exceptional nature of the circumstances in which these remains have been entombed and preserved, only a comparatively small proportion of the various tribes of plants and animals living at any time upon the earth is likely to be fossilised. Those organisms which contain hard parts are best fitted for becoming fossils. The original substance of the organism may, in rare cases, be preserved; more usually the organic matter is partially or wholly removed. Sometimes a mere cast of the plant or animal in amorphous mineral matter retains the outward form without any trace of the internal structure. In other instances, true petrifaction has taken place, the organic structure being reproduced in calcite, silica, or other mineral by molecular replacement. Fossils are of the utmost value in geology, inasmuch as they indicate (1) former changes in geography, such as the existence of ancient land-surfaces, lakes, and rivers, the former extension of the sea over what is now dry land, and changes in the currents of the ocean; (2) former conditions of climate, such as an Arctic state of things as far south as Central France, where bones of reindeer and other Arctic animals have been found; (3) the chronological sequence of geological formations, and, consequently, the succession of events in geological history, each great group of strata being characterised by its distinctive fossils. This is the most important use of fossils. Having ascertained the order of superposition of fossiliferous rocks, that is, the order in which they were successively deposited, and having found what are the characteristic fossils of each subdivision, we obtain a guide by which to identify the various rock-groups from district to district, and from country to country. By means of the evidence of fossils the stratified rocks of the Geological Record have been divided into sections and subsections, to which names are applied that have now come to designate not merely the rocks and their fossils, but the period of geological time during which these rocks were accumulated and these fossils actually lived. |