PALAEONTOLOGY The general principles of palaeontology are discussed in a sepa rate article (q.v.), but for the sake of continuity and at the risk of slight repetition it will be well here to consider certain points having an important bearing on stratigraphical investigation.
Life-provinces.—It is notorious that at the present time the animals and plants of different parts of the world differ widely, the variations being largely a matter of climate and environment. Many distinct life-provinces have been demarcated, both zoologi cal and botanical. Now there is every reason to believe that the same state of things held in the past, and likewise many palaeo zoological provinces have been recognized, though naturally in less detail than the modern ones. Consequently, it follows that con temporaneous deposits in different parts of the world need not contain the same fossils ; but, just as at present a few species are almost world-wide, so they were in the past. Two contempo raneous faunas can usually be linked up by some forms common to both, especially among marine animals.
Biological Facies.—We now come to the important considera tion of the value of fossils as indicators of geographical conditions. It is hardly necessary to point out that marine animals are found in marine deposits, land animals and plants in freshwater and terrestrial deposits, and so on ; the real matter at issue is : to what extent slightly different assemblages of contemporaneous fossils can be taken as indicators of geographical conditions. This problem is exactly the same as that discussed in connection with lithological variations in rocks, and denotes the existence of biological facies.
For the sake of simplicity attention may be confined to marine conditions. Everybody knows that marine animals vary much in their habits; some remain fixed to rocks or buried in the mud and sand of the sea-floor, some swim freely, while others float passively as carried by tides and currents, some live between tide-marks and are above water for a considerable part of each day, others must be always submerged, some in shallow, some in deep water and so on. It is clear that among primeval creatures a similar state of things prevailed, as shown by analogy, and among extinct forms conclusions as to mode of life can be drawn from structure. Hence by a study of fossils it is possible to draw up a scheme of facies, just as in the case of rocks. In actual practice the two are always combined, yielding precise information as to geograph ical and often as to climatic conditions.
In the foregoing sections a brief sketch has been given of the materials with which stratigraphical geology has to work ; the "tools of the trade." Another point which scarcely needs elabora tion is the importance of maps. Maps and sections are the only possible means of representing the results in a convenient form, capable of easy multiplication and distribution. A solid model is of course the best, but such are necessarily cumbrous, and suited only to a limited number of cases.
It may be said then that in the simplest terms the task of the stratigraphical geologist is first to work out the present structure of the earth, and from the facts thus ascertained to draw deduc tions as to events in the past. According to personal idiosyncra sies these deductions may take somewhat different forms; palae ontological, petrological, palaeogeographical and so on, but on the whole the aim is historical, the elucidation of the history of the earth. Over large areas this history has been worked out in very considerable detail, and the remainder of this article will be chiefly occupied by a very brief and generalized summary of the results so far attained. Much still remains to be done in the remoter and less developed regions, but it may probably be said with safety that the broad lines are now firmly established.
In treating a historical subject it is usually advisable to begin at the beginning, but, as before explained, this is just what geology cannot do. We are constrained therefore to adopt the large and unsatisfactory category of Pre-Cambrian rocks for our lowest division, fully aware that this group extends indefinitely down ward and probably covers a vastly greater period than all the later formations put together ; for while it is generally agreed by astronomers and cosmogonists that whatever may have been the origin of the earth it must have passed through a molten stage, and therefore must have had a "first crust," geologists hold that it is very improbable that any of this first crust has survived and that it was probably broken up and re-melted over and over again before a permanent solid covering for the molten interior was developed. It is true that the oldest known rocks are highly crys talline, but they always show signs of having been derived from older rocks rather than formed by direct crystallization from a fused state. In the later part of Pre-Cambrian time at any rate, conditions seem to have been very similar to what they are now, except for the absence or apparent absence of living beings; there is reason to believe that the earlier developments of life on any considerable scale were confined to the sea, for there is little or no evidence for the existence of land-dwellers till a later date than some highly organized marine, faunas ; but, though their re mains have not been found, there is no proof that there were not lowly animals and plants on land as early as in the sea.
Methods of Subdivision.—The next point to be considered is the general principle to be adopted in the choice of subdivisions and nomenclature for the stratified rock-series. It will no doubt already be apparent that geological history is not a smooth and continuous sequence of uniform conditions; there are breaks in the continuity and these are naturally adopted to mark the periods. Now the most obvious kind of break is an unconformity, and most of the major time-divisions are determined by these. In the un f ossilif erous rocks this is the only kind of subdivision that is possible; but when fossils are present the rocks may also be sub divided according to the palaeontological succession, and this method is now very extensively adopted, as being more scientific than the purely physical method. The ideal classification would be a combination of both, but unfortunately the well marked lines do not always coincide in the two scales. A change of fauna does often coincide with a physical break, but these physical breaks are not always contemporaneous in different places. Upheavals due to earth-movements crawl slowly over the world, as it were, and there is often time for a considerable change of fauna during their progress, so that the break may come at one life-zone in one place and at another life-zone in another place. This is a crude illustration of the kind of difficulty that stratigraphy has to contend with, but, when treated on the right lines, is very illumi nating as to the history and geography of the past.
Space does not permit us to give here an account of the stratig raphy of the whole world ; general summaries of the particular formations will be found in special articles and a sketch of the gen eral geology is given in the articles dealing with each country; so all that will be attempted here is a brief outline, by way of exam ple, of the principles adopted in subdividing the strata of the Brit ish Isles; the general geological history of North America is very similar, while that of Africa, India and Australia for example, are very different, and in some sense antithetical, especially in the later periods. There is evidence that in the earliest times climatic and other conditions were more uniform over the whole earth than they are now, and some of the earliest marine faunas were very widely spread.
Taking the British Isles as a whole, the oldest rocks are found in the north and west, the newest in the south and east : this means that in general the country is tilted towards the south-east, like a pile of books that has fallen over. In the extreme north-west of Scotland and in the Outer Hebrides are the oldest rocks of all, the Lewisian gneiss, a highly crystalline series mainly composed of igneous rocks; this is succeeded, with a very strongly marked un conformity, by the Torridon Sandstone, a quite normal series of sedimentary rocks, evidently formed under arid conditions. This again is followed unconformably by the lowest Cambrian fossil iferous rocks. Here then we have a very simple but typical suc cession : Pre-Cambrian rocks in two well-marked and strongly contrasted types, the later being clearly of continental origin, succeeded by a marine transgression. Here in this particular region the succession is cut off short, owing to later disturbances which need not now be discussed ; but Cambrian rocks are found again in Wales and the Western Midlands, so that the succession can be picked up and continued. The marine transgression soon be came very extensive and an immensely thick series of marine strata were laid down, with minor breaks locally : this state of affairs lasted a long time and the rocks are now divided into three systems, Cambrian, Ordovician and Silurian. These constitute the First Marine period. Towards the end of Silurian times uplift and disturbance occurred, culminating in the Caledonian moun tain-building movements, which formed a great northern land area, leaving sea where Devonshire now is. On this great conti nent, named by Walther the "Old Red Northland," the desert deposits and lakes of the Old Red Sandstone were formed. while in the south were laid down the marine strata appropriately named Devonian, which extend underground to Belgium and Germany. In the north this constituted the Second Continental Period, and was accompanied by an immense development of igneous rocks (the Highland and Lake District granites, and the lavas of the Ochils, Sidlaws and Cheviots). After this the sea again crept north, spreading limestones far over England, with shales and sandstones and even coal in the north, indicating a shore line in Scotland. This constitutes the Second Marine Period of the Lower Carboniferous. This did not last very long: north-western Europe was soon converted into a low-lying, swampy land area on which the Coal-measures were formed in the Upper Carboniferous. Then the continent rose and extended, so that desert conditions again set in and continued during the Permian and Trias. About the end of the Carboniferous there was another phase of mountain building and igneous activity, when the Pennine Hills and the Mendips were formed and granites rich in metals were intruded in Devon and Cornwall. Salt lakes were also numerous. This was the Third Continental Period. After a long while the sea again broke into the desert basins of the Trias and rapidly extended, ushering in the Third Marine Period, which comprises the rocks of the Jurassic and Cretaceous, culminating in the great Chalk sea, which spread far into France and Germany, but was cut off from the Mediterranean region by a barrier. Soon after mountain building became acute in central and southern Europe, leading to the uprise of the Alps, and slight earth-waves were propagated far to the north-west, again bringing in land conditions over most of the area, and this state of affairs continued for the rest of geological time, constituting the Tertiary and Recent, the Fourth Continental Period : accompanying these movements were violent volcanic eruptions in N.E. Ireland and Western Scotland.
Such in the baldest outline is the geological history of the British Isles, which may now be re-stated in the form of a table, from which certain generalizations will be drawn : In connection with this table several points arise for com ment. In the first place it brings out with great clearness the close relationship between unconformities, earth-movements and igne ous activity. The major unconformities are due to the moun tain-building movements, and minor unconformities, not here mentioned, are of course due to similar causes on a smaller scale : for example there are minor unconformities between Cambrian and Ordovician and Ordovician and Silurian : also in many places between Jurassic and Cretaceous. Locally again there are smaller unconformities and breaks within the systems, leading to minor subdivisions. In other cases the systems are subdivided by litho logical variations, or by changes of fauna (zones). Both methods are now usually followed and often lead to inconsistent results, as explained in the section of this article on the biological side of stratigraphy.
Another important principle arising from a study of this table is that the beginning and end of a continental period differ in character. The beginning of a period of disturbance is indicated by an interruption in the regular deposition of marine strata, usually local at first, then becoming general, culminating in fold ing, denudation and a marked unconformity, so that the strata of the continental type rest on the eroded edges of the older rocks ; but at the end of the period either the land gradually sinks or the sea-level rises, resulting in a quiet marine transgression, with well-marked overlap, and a continental period gradually slides into a marine period without any marked convulsion of nature. A good instance is afforded by the gradual transition from the Old Red Sandstone to the marine Carboniferous limestone. In many places the passage from one to the other is perfectly conformable, and the boundary can only be drawn on lithological or palaeontological grounds. This is a general principle of wide application. The lowest division, the Pre-Cambrian formations, happen to be rather poorly represented in the British Isles; in a similar table for Canada or India they would show up much more conspicuously, and in a South African table it would be necessary to show five or six systems of almost certainly Pre-Cambrian age, though un fortunately there the systems from Cambrian to Silurian, if pres ent at all, contain no fossils. In Canada likewise a large number of undoubted Pre-Cambrian systems would have to be shown, though opinion is as yet by no means settled as to how they should be classified. But the British Isles happen to contain an unusually complete series of f ossilif erous formations and therefore afford an excellent general example.
Nomenclature.—An inspection of any list of geological forma tions will at once show that the names adopted are purely arbi trary. Many of them are geographical, derived from localities where the rocks are well represented (Torridonian, from Loch Torridon, in N.W. Scotland; Devonian; Jurassic, from the Jura Mts., etc.) ; sometimes they are derived from ancient names for the country or for its inhabitants (Cambrian, the Roman name for Wales, Ordovician and Silurian from ancient tribes inhabiting the Welsh borderland), while another group comes from lithological characters (Old Red Sandstone ; Carboniferous=coal-bearing; Cretaceous from creta, Lat. for chalk) . "Trias" is derived from a three-fold division of the system on the continent of Europe, which is not seen in England. The Tertiary again is subdivided into Eocene, Oligocene, Miocene, Pliocene (see these headings), names of Greek origin founded on the numerical proportions of living species in each.
Most of these names are in world-wide use though they are not always very applicable in distant countries ; it is when we come to those of the smaller subdivisions that chaos sets in, many of them being of purely chance origin, especially among the older ones. The British rocks were first roughly classified, as before men tioned, by William Smith, about the beginning of the last century; and many of his rough-and-ready names have stuck and have obtained world-wide currency, sometimes in their original form, sometimes slightly altered. They are usually descriptive of the character of the rocks and many have a geographical name tacked on, such as Oxford Clay, Stonesfield Slate, Lincolnshire Lime stone. A few are derived from English dialect words, as are probably, Lias and Gault. Such names as Coal-measures, Lower Greensand and Magnesian Limestone explain themselves ; in cer tain cases old names have been more or less classicized or mod ernized, taking such forms as Corallian, Callovian (from Kella ways Rock) and so on, and a large number of new names have been constructed on similar lines.
Attempts have been made, especially by palaeontologists, to sweep away these old historic names and to substitute an entirely new and "scientific" classification. On logical and utilitarian grounds such a procedure would perhaps be justified, but very many geologists, especially field geologists, would deeply regret their disappearance, since they do at any rate indicate where and how stratigraphical geology took its rise, and they serve as a lasting memorial to William Smith, Adam Sedgwick, Sir Roderick Murchison and the other founders of stratigraphical geology. In connection with nomenclature one more point remains in need of explanation ; namely, the meaning of the names Palaeozoic, Mesozoic and Cainozoic, used in this table to indicate three out of the four major subdivisions adopted. Etymologically the words mean ancient, middle and recent life, and thus of course refer to the character of the fossils found in the rocks : each is the subject of a special article (q.v.) and the subject need not be elaborated here. Sometimes these names are replaced by Primary, Secondary and Tertiary respectively ; the first is little used, Secondary and Mesozoic are employed almost indiscriminately, while Tertiary is perhaps even commoner than Cainozoic. It is all a matter of taste, as the connotations of the two sets of names are identical.