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ORIGIN OF THE TERM TERTIARY—LOWER, MIDDLE, AND UPPER, OR EOCENE, MIOCENE, AND TLIOCENE TERTIARIES — THEIR MINERAL COMPOSITION AND SUCCESSION—DIFFERENT IN DIFFERENT BASINS OR AREAS OF DEPOSIT—FLORA AND FAUNA OF THE RESPECTIVE SUBDIVISIONS—PHYSICAL CONDITIONS UNDER WHICH THESE GREW AND WERE DEPOSITED—GIGANTIC AND INTERMEDIATE FORMS OF EOCENE MAMMALS—APPROXIMATION TO EXISTING DISTRIBUTIONS IN MIOCENE AND PLIOCENE TIMES—ECONOMIC PRODUCTS OF THE SYSTEM.
When the earlier geologists arranged the stratified rocks of the earth-crust into Primary, Secondary, and Tertiary, they closed the Secondary with the Chalk, and regarded as Tertiary all the sediments that occur above that formation. In their estimation Tertiary strata were comparatively limited, and they had no conception of the extent, thickness, and variety of sediments, or of the long periods which this thickness and variety of deposits must necessarily imply. To them primary was equivalent to universal; secondary was still very extensive, though not universal; but the tertiaries were mere local patches occupying shallow depressions in the older formations. As research went forward, however, modern geologists began to discover that tertiary strata differed widely among themselves both in composition and fossil contents, and that as a whole they were entitled to be ranked in schemes of classification as a system of sedimentary deposits. It was also found that while all these sediments, or rather formations of sediments, were highly fossiliferous, they were covered over, at least in the greater portion of the northern hemisphere, by thick masses of bouldery clay and gravel, devoid, or all but devoid, of fossils; and this bouldery clay was considered as necessarily limiting or closing the system. In this way the Tertiary of the earlier geologists became to be broken up into Tertiary and Quaternary—the former embracing all the strata that lie between the Chalk and Boulder - Clay, and the latter the boulder-clay and all those superficial accumulations that have since been formed, or are still in process of formation, by the ordinary agencies of nature. Tabulating this arrangement, we have the
Quaternary, or Post - Tertiary, embracing all the superficial clays and muds, the sands and gravels, the peat-mosses, swamp - growths, coral-reefs, lake-silts, estuary - silts, and sea-silts, with the boulder-clay beneath; and the
Tertiary, all the regularly stratified clays, sands, and gravels, marly limestones, gypsums, and lignites or brown-coals, that lie between the Boulder-Clay and the Chalk formations.
In other words, the Quaternary or Post-Tertiary embraces the rocks and records of the current epoch; the Tertiary, the rocks and records of a period that long preceded. And this not a brief period, but one of long continuance—so long, that during its currency there were many oscillations of sea and land, many extinctions of genera and species, and many introductions of other and newer races.
As might be expected in a system embracing a long period of time, the rocks and fossils of the earlier tertiaries differ considerably from those of the later; and hence their familiar arrangement into lower, middle, and upper; or into eocene, miocene, and pliocene, if we take, as Sir Charles Lyell has done, the relative proportions of recent and fossil shells which occur in the successive stages.* But whatever the nomenclature we adopt, it is clear that the clays, sands, gravels, marls, limestones, gypsums, and lignites which constitute the tertiary system are the varied sediments of lakes, estuaries, and shallow seas; and that in general the extent and boundaries of these areas of deposit are more apparent than those of the earlier systems. Indeed, many of the tertiary deposits occupy limited spaces in the existing continents, and thus we are led to infer that something like the present distribution of sea and land had then begun to prevail. At all events, much of the existing dry land was then above water, and supplied the material for the sediments, as well as the habitats for the terrestrial flora and fauna of the period. Of course it is not contended that the continents had then assumed their present outlines, nor that many lands then existing are not now submerged; but it is indicated that there is considerable relationship between Tertiary and Recent times, and that the fossils of the Old World tertiaries are more akin to the living plants and animals of the Old World than to those of the New; while, on the other hand, the tertiary fossils of the New World bear a striking resemblance to the plants and animals that still flourish there. In other words, we are approaching in tertiary times more nearly to the existing ordainings of nature, and may therefore expect a closer resemblance in all the phenomena, physical and vital, than it was possible to trace between the present and any of the remoter epochs. The reader who will take the trouble to consult a geological map of Europe will see at a glance that much of the continent was occupied by seas and lakes during the Tertiary period, and that it was not till towards the close of the epoch that as a land-mass it began to assume its existing configuration and dimensions. As with Europe, so to a great extent with Asia and Africa, but notably so with North and South America. The great land-masses had then been merely blocked out, as it were; their existing aspects are the results of Tertiary and Quaternary modifications.
* Taking a hundred shells from the lower, a hundred from the middle, and a hundred from the upper tertiaries of the London and Paris basins, Sir Charles found that only a small percentage (3 to 5) of existing species occurred in the lower, hence eocene (Gr. eos, dawn; kainos, recent), or dawn of existing things; that the number of existing species was somewhat less than the extinct (25 to 40) in the middle, hence miocene (melon, less), that is, less recent than the existing; and that in the upper the existing species exceeded the extinct (70 to 90), hence pliocene (pleion, more), that is, more recent than the middle or lower divisions.
As Rocks, there is nothing difficult to comprehend either in the nature or composition of the tertiary sediments. In one basin or area of deposit we have alternations of sands, gravels, clays, and lignites; in another, sands, gravels, clays, limestones, gypsums, and lignites; and in a third, clays, lignites, marls, and interstratified overflows of lava or showers of volcanic ashes. All these alternations are well displayed in the tertiary basins of London, Hampshire, Paris, Auvergne, the Lower Ehine, and Vienna; and the observer has no greater difficulty in comprehending the nature of these successions than he has in interpreting the sediments of any lake or estuary of the present day. In some basins the limestones may be hard and compact, or even silicious, like the burr-stone of Paris; in others, the sands may be consolidated into sandstones; in some, the lignites may be peaty and woody, while in others they are scarcely distinguishable from ordinary coal; but taking them all in all, the tertiary strata present few difficulties either as regards composition or the agencies concerned in their formation. In some of the areas of deposit, as evidenced by their fossils, the strata are strictly marine, in others they are fresh-water or lacustrine, and in some, again, there is an admixture of sea and river silts, which are consequently regarded as estuarine or fluvio-marine. Just as at the present day such seas as the Adriatic, Euxine, and Caspian are receiving their sediments contemporaneously with the deltic deposits of the Po, Danube, and Volga, and these again contemporaneously with the silts of the Venetian and Hungarian lakes; so in tertiary times the same sort of operations went simultaneously forward, and thus we find throughout the system a variety not only of sediments, but of fossils, though all belonging to one great and continuous period of worldhistory.
There were also throughout the deposition of the tertiary strata abundant manifestations of volcanic activity, and in few tertiary districts are there wanting the cones, crateriform hills, lava overflows, ash-beds, and trachytic tufas that mark the comparative recentness of their production. In Central France, the Lower Ehine, Italy, Hungary, Greece, Western and Central Asia, Australia, and New Zealand, such evidences are everywhere abundant, and even in our own islands the basalts of Antrim and the trap-tuffs of Mull present their concurrent testimony. Less crystalline than the greenstones, felstones, and porphyries of the secondary and primary periods, and more compact than the lavas and cinder-beds of the present day, the tertiary traps are readily distinguished; and even where most consolidated, their age is easily determined by the associated strata. In Auvergne, the Lower Ehine, and Greece, they are found with beds of eocene and miocene age; in Australia, basaltic overflows cover auriferous gravels of pliocene date; and in New Zealand they spread over lignites of perhaps still later origin.
As a Life-period the tertiary system stands in remarkable contrast with all that we know of the preceding formations.