XV. THE TERTIARY SYSTEM, EMBRACING THE EOCENE, MIOCENE, PLIOCENE, AND PLEISTOCENE GROUPS. 134. THE earlier geologists, in dividing the stratified crust into primary, secondary, and tertiary formations, regarded as tertiary all that occurs above the Chalk. The term is still retained, but the progress of discovery has rendered it necessary to restrict and modify its meaning. Even yet the limits of the system may be said to be undetermined- —some embracing under the term all that lies between the Chalk and Boulder-drift, others including the drift and every other accumulation in which no trace of man or his works can be detected. Palæontologically speaking, much might be said in favour of both views, but the difficulty of unravelling the relations of many clays, sands, and gravels, makes it safer to adopt in the mean time a somewhat provisional arrangement. We shall therefore treat as TERTIARY all that occurs above the chalk to the close of the drift, and as POST-TERTIARY every accumulation which appears to have been formed since that period. Adopting this arrangement, we have in England the following intelligible subdivisions :— POST-TERTIARY. { RECENT and SUPERFICIAL ACCUMULATIONS occurring above the boulder-drift. PLEISTOCENE......Boulder-drift. PLIOCENE ..........Mammaliferous and Red crag. ¿TERTIARY. MIOCENE. Coralline crag. .Strata of London and Hampshire basins. By adopting this view we get rid of certain anomalies connected with the boulder-drift, while there will be no difficulty in removing the pleistocene to the post-tertiary system, should subsequent discoveries render such a transposition necessary. 135. The organic types of the system above indicated are all Cainozoic-that is, are all more or less allied to, or even identical with, many existing genera. As at the close of the Paleozoic, cycle graptolites, trilobites, eurypterites, pterichthys, coccosteus, cephalaspis, megalichthys, sigillaria, lepidodendron, and other forms of ancient life had passed away, so, at the close of the Mesozoic, the ichthyosaurus, plesiosaurus, pterodactyle, palæoniscus, ammonites, and encrinites disappeared, and their place was taken by other and more recent-like forms. We now find among vegetables evidence of true exogenous timber-trees (that is, trees which increase by external layers of growth, like the oak, beech, and elm); a large percentage of the corals and shells are identical with those of existing seas; the reptiles are carapaced turtles, tortoises, and crocodiles; the fishes are chiefly ctenoids and cycloids with equally-lobed tails; birds of existing families are by no means rare; and examples of mammalia of all orders up to the highest, save man, have been detected. It is true that certain genera and species discovered in tertiary strata are not to be found beyond the limits of the pliocene group; and it is this extinction of many peculiar forms that warrants the separation of the post-tertiary from the tertiary system. The groups are founded upon this perceptible approach to existing species-taking the fossil shells as the index. Thus, eocene (Gr. eos, the dawn, and kainos, recent) implies that the strata of this group contain only a small proportion of living species, which may be regarded as indicating the dawn of existing things; miocene (meion, less) implies that the proportion of recent shells is less than that of extinct; pliocene, (pleion, more), that the proportion of recent shells is more or greater than that of the extinct; and pleistocene (pleiston, most), that the shells of this group are mostly those of species inhabiting the present seas. Eocene, Miocene, and Pliocene Groups. 136. We arrange these groups under one head, because they are all evidently sedimentary deposits resulting from the ordinary operations of water, and because they are all less or more fossiliferous, and thus give evidence of the geographical conditions of the world during the period of their formation. The case is different with the pleistocene or boulder-drift group, which is clearly not an ordinary sedimentary deposit, and which, with rare exceptions, is altogether destitute of fossils. Confining our remarks to the three lower groups, we find the composition and succession of their strata so extremely varied and irregular, that it is next to impossible to give anything like a generally applicable descrip tion. This much may be said, that their areas are usually well defined, as if originally deposited in inland seas or estuaries; that they give evidence of frequent alternations of marine with fresh-water sediments; and, on the whole, are less consolidated than the rocks of older systems. They consist for the greater part of clays, sands, and gravels, with interstratified limestones, calcareous grits, marls, and occasional layers of lignite. With respect to the composition and succession of their strata, the following synopsis of the English tertiaries will convey a better idea than any detailed description : PLIO CENE. MIOCENE. EOCENE. MAMMALIFEROUS CRAG of Norfolk and Suffolk.-Consist- CORALLINE CRAG.-A mass of shells and corals in calcare- FLUVIO-MARINE BEDS of Hampshire and Isle of Wight. LONDON CLAY.-A brown or dark-blue or blackish tenacious BOGNOR BEDS.-Occur towards the base of the London clay, and consist of calcareous and silicious nodules, or of coarse, green indurated sand, with numerous marine shells. PLASTIC CLAY AND SANDS.-Composed of sand, shingle, mottled clays, and loam, with beds of rolled flints and marine shells. The above may be taken as sufficiently descriptive of the English tertiaries; and for the sake of comparison, we subjoin a section (in descending order) of the strata in the Paris basin, which are usually regarded as the equivalents of the English eocene :— UPPER. Upper fresh-water limestone marls and silicious millstone (burrstone). Upper marine-sands or Fontainebleau sandstone and sands. MIDDLE. LOWER. Lower fresh-water limestone and marl, or gypseous series. Lower sands, with marine shelly beds (lits coquillier). 137. As with the Paris and English deposits, so with the other tertiary basins of southern France, Spain, Austria, Hungary, Italy, &c.—all of them exhibiting an irregular succession of clays, sands, limestones, marls, gypsum, and lignites, which, when examined lithologically and palæontologically, are clearly referable to the same period of formation. Among the most remarkable features of foreign tertiaries are the infusorial and nummulitic strata the former constituting such rocks as the "tripoli" of Bohemia and Virginia, and the latter the "nummulitic limestones," so abundant in southern Europe, Egypt, and Asia. The tripoli consists almost entirely of the silicious coverings of infusorial animalcules, and is often of great thickness, as at Richmond in Virginia, where it is nearly thirty feet; and the nummulitic limestone, which is composed of coin-shaped (nummus, a coin) foraminiferous shells, is perhaps the most important of ter tiary strata. Respecting this limestone, which was till recently regarded as belonging to the cretaceous system rather than to the base of the eocene tertiaries, Sir Charles Lyell remarks that "it often attains a thickness of many thousand feet, and extends from the Alps to the Apennines. It is found in the Carpathians, and in full force in the north of Africa-as, for example, in Algeria and Morocco. It has also been traced from Egypt into Asia Minor, and across Persia by Bagdad to the mouths of the Indus. It occurs not only in Cutch, but in the mountain-ranges which separate Scinde from Cabul; and it has been followed eastward into India." From India its course has been recently traced onward to the Philippine Islands, thus demonstrating the existence of one of the most gigantic accumulations from the smallest of organic causes. A similar deposit of great extent occurs in North America, but in this case the organism is the orbitolite (globe-shaped), and hence it is there known as the orbitoidal limestone. 138. With respect to the extent and distribution of the lower tertiaries—laying aside the nummulitic limestone, which is in some respects a peculiar and unique development-we have as yet no certain knowledge. As there is often no perceptible mineral distinction between many clays, sands, and gravels, it is only by their imbedded fossils that geologists can determine their tertiary or post-tertiary character. Many accumulations at present regarded as superficial may be found hereafter to be of older date; and thus it becomes difficult to fix with certainty the geographical limits of the system. So far as Europe is concerned, tertiary deposits have received considerable attention, and their area has been found to be much more extensive than was at one time supposed. In general, the deposits occupy well-defined tracts or basins; hence the frequent reference in works on geology to the "London basin," " 'Hampshire basin," "Paris basin," "Vienna basin," and other tertiary tracts in Europe. As far as discovery has gone, there are few countries in Europe where tertiary strata have not been detected; and while we regard those of England, France, Austria, and Italy as typical, we must ever bear in mind that considerable modifications may require to be made, as the tertiaries of India and North and South America come to be more closely examined. One important fact must not be lost sight of in drawing any general conclusions from the distribution of tertiary deposits-viz., that as the fauna and flora of the period approach in character the fauna and flora of existing nature, and that as the plants and animals of Europe, India, Australia, and South America, &c., all differ widely from each other, so may we expect similar differences among the fossil remains of these distant regions. And this, as will afterwards be seen, is fully borne out-the tertiary mammals of South America resembling the sloths, armadilloes, ant-eaters, and alpacas of that continent; those of Australia its marsupial kangaroos and opossums; while those of the Old World have more immediate relationship to its elephants, rhinoceroses horses, deer, and oxen. And here it may be remarked that the student cannot too early direct his attention to the laws which regulate the distribution of life on the globe, and be able to distinguish clearly between identity and representation of species. During the Paleozoic and Mesozoic epochs there appears to have been a greater identity of species over wide areas; during the present period the areas are more circumscribed, and the species in one K |