A Manual of Elementary Geology Part 18

_Groups of Fossiliferous Strata observed in Western Europe, arranged in what is termed a descending Series, or beginning with the newest._ (_See a more detailed Tabular view_, pp. 360. 365.)

1. Post-Pliocene, including those of the Recent, or human period.

2. Newer Pliocene, or Pleistocene. } 3. Older Pliocene. } Tertiary, Supracretaceous[103-A], 4. Miocene. } or Cainozoic.[103-B]

5. Eocene. }

6. Chalk. } 7. Greensand. } 8. Wealden. } 9. Upper Oolite. } Secondary, or Mesozoic.[103-B]

10. Middle Oolite. } 11. Lower Oolite. } 12. Lias. } 13. Trias. }

14. Permian. } 15. Coal. } 16. Old Red sandstone, or Devonian. } Primary fossiliferous, 17. Upper Silurian. } or paleozoic.[103-B]

18. Lower Silurian. } 19. Cambrian and older fossiliferous strata. }

It is not pretended that the three princ.i.p.al sections in the above table, called primary, secondary, and tertiary, are of equivalent importance, or that the eighteen subordinate groups comprise monuments relating to equal portions of past time, or of the earth's history. But we can a.s.sert that they each relate to successive periods, during which certain animals and plants, for the most part peculiar to their respective eras, have flourished, and during which different kinds of sediment were deposited in the s.p.a.ce now occupied by Europe.

If we were disposed, on palaeontological grounds[103-C], to divide the entire fossiliferous series into a few groups less numerous than those in the above table, and more nearly co-ordinate in value than the sections called primary, secondary, and tertiary, we might, perhaps, adopt the six groups or periods given in the next table (p. 104.).

At the same time, I may observe, that, in the present state of the science, when we have not yet compared the evidence derivable from all cla.s.ses of fossils, not even those most generally distributed, such as sh.e.l.ls, corals, and fish, such generalizations are premature, and can only be regarded as conjectural or provisional schemes for the founding of large natural groups.

_Fossiliferous Strata of Western Europe divided into Six Groups._

1. Post Pliocene and } from the Post-Pliocene to the Tertiary } Eocene inclusive.

2. Cretaceous { from the Maestricht Chalk to the Lower { Greensand inclusive.

3. Oolitic from the Wealden to the Lias inclusive.

4. Tria.s.sic { including the Keuper, Muschelkalk, and { Bunter Sandstein of the Germans.

5. Permian, Carboniferous, } including Magnesian Limestone (Zechstein), and Devonian } Coal, Mountain Limestone, and } Old Red sandstone.

6. Silurian and Cambrian } from the Upper Silurian to the oldest } fossiliferous rocks inclusive.

FOOTNOTES:

[103-A] For tertiary, Sir H. De la Beche has used the term "supracretaceous," a name implying that the strata so called are superior in position to the chalk.

[103-B] Professor Phillips has adopted these terms: Cainozoic, from +kainos+, _cainos_, recent, and +zoon+, _zoon_, animal; Mesozoic, from +mesos+, _mesos_, middle, &c.; Paleozoic, from +palaios+, _palaios_, ancient, &c.

[103-C] Palaeontology is the science which treats of fossil remains, both animal and vegetable. Etym. +palaios+, _palaios_, ancient, +onta+, _onta_, beings, and +logos+, _logos_, a discourse.

CHAPTER X.

CLa.s.sIFICATION OF TERTIARY FORMATIONS.--POST-PLIOCENE GROUP.

General principles of cla.s.sification of tertiary strata--Detached formations scattered over Europe--Strata of Paris and London--More modern groups--Peculiar difficulties in determining the chronology of tertiary formations--Increasing proportion of living species of sh.e.l.ls in strata of newer origin--Terms Eocene, Miocene, and Pliocene--Post-Pliocene strata--Recent or human period--Older Post-Pliocene formations of Naples, Uddevalla, and Norway--Ancient upraised delta of the Mississippi--Loess of the Rhine.

Before describing the most modern of the sets of strata enumerated in the tables given at the end of the last chapter, it will be necessary to say something generally of the mode of cla.s.sifying the formations called tertiary.

The name of tertiary has been given to them, because they are all posterior in date to the rocks termed "secondary," of which the chalk const.i.tutes the newest group. These tertiary strata were at first confounded, as before stated, p. 91., with the superficial alluviums of Europe; and it was long before their real extent and thickness, and the various ages to which they belong, were fully recognized. They were observed to occur in patches, some of freshwater, others of marine origin, their geographical area being usually small as compared to the secondary formations, and their position often suggesting the idea of their having been deposited in different bays, lakes, estuaries, or inland seas, after a large portion of the s.p.a.ce now occupied by Europe had already been converted into dry land.

The first deposits of this cla.s.s, of which the characters were accurately determined, were those occurring in the neighbourhood of Paris, described in 1810 by MM. Cuvier and Brongniart. They were ascertained to consist of successive sets of strata, some of marine, others of freshwater origin, lying one upon the other. The fossil sh.e.l.ls and corals were perceived to be almost all of unknown species, and to have in general a near affinity to those now inhabiting warmer seas. The bones and skeletons of land animals, some of them of large size, and belonging to more than forty distinct species, were examined by Cuvier, and declared by him not to agree specifically and for the most part not even generically, with any hitherto observed in the living creation.

Strata were soon afterwards brought to light in the vicinity of London, and in Hampshire, which, although dissimilar in mineral composition, were justly inferred by Mr. T. Webster to be of the same age as those of Paris, because the greater number of the fossil sh.e.l.ls were specifically identical. For the same reason rocks found on the Gironde, in the South of France, and at certain points in the North of Italy, were suspected to be of contemporaneous origin.

A variety of deposits were afterwards found in other parts of Europe, all reposing immediately on rocks as old or older than the chalk, and which exhibited certain general characters of resemblance in their organic remains to those previously observed near Paris and London. An attempt was therefore made at first to refer the whole to one period; and when at length this seemed impracticable, it was contended that as in the Parisian series there were many subordinate formations of considerable thickness which must have acc.u.mulated one after the other, during a great lapse of time, so the various patches of tertiary strata scattered over Europe might correspond in age, some of them to the older, and others to the newer, subdivisions of the Parisian series.

This error, although most unavoidable on the part of those who made the first generalizations in this branch of geology, r.e.t.a.r.ded seriously for some years the progress of cla.s.sification. A more scrupulous attention to specific distinctions, aided by a careful regard to the relative position of the strata containing them, led at length to the conviction that there were formations both marine and freshwater of various ages, and all newer than the strata of the neighbourhood of Paris and London.

One of the first steps in this chronological reform was made in 1811, by an English naturalist, Mr. Parkinson, who pointed out the fact that certain sh.e.l.ly strata, provincially termed "Crag" in Suffolk, lay decidedly over a deposit which was the continuation of the blue clay of London. At the same time he remarked that the fossil testacea in these newer beds were distinct from those of the blue clay, and that while some of them were of unknown species, others were identical with species now inhabiting the British seas.

Another important discovery was soon afterwards made by Brocchi in Italy, who investigated the argillaceous and sandy deposits replete with sh.e.l.ls which form a low range of hills, flanking the Apennines on both sides, from the plains of the Po to Calabria. These lower hills were called by him the Subapennines, and were formed of strata of different ages, all newer than those of Paris and London.

Another tertiary group occurring in the neighbourhood of Bordeaux and Dax, in the south of France, was examined by M. de Basterot in 1825, who described and figured several hundred species of sh.e.l.ls, which differed for the most part both from the Parisian series and those of the Subapennine hills. It was soon, therefore, suspected that this fauna might belong to a period intermediate between that of the Parisian and Subapennine strata, and it was not long before the evidence of superposition was brought to bear in support of this opinion; for other strata, contemporaneous with those of Bordeaux, were observed in one district (the Valley of the Loire), to overlie the Parisian formation, and in another (in Piedmont) to underlie the Subapennine beds. The first example of these was pointed out in 1829 by M. Desnoyers, who ascertained that the sand and marl of marine origin called Faluns, near Tours, in the basin of the Loire, full of sea-sh.e.l.ls and corals, rested upon a lacustrine formation, which const.i.tutes the uppermost subdivision of the Parisian group, extending continuously throughout a great table-land intervening between the basin of the Seine and that of the Loire. The other example occurs in Italy, where strata, containing many fossils similar to those of Bordeaux, were observed by Bonelli and others in the environs of Turin, subjacent to strata belonging to the Subapennine group of Brocchi.

Without pretending to give a complete sketch of the progress of discovery, I may refer to the facts above enumerated, as ill.u.s.trating the course usually pursued by geologists when they attempt to found new chronological divisions. The method bears some a.n.a.logy to that pursued by the naturalist in the construction of genera, when he selects a typical species, and then cla.s.ses as congeners all other species of animals and plants which agree with this standard within certain limits.

The genera A. and C. having been founded on these principles, a new species is afterwards met with, departing widely both from A. and C., but in many respects of an intermediate character. For this new type it becomes necessary to inst.i.tute the new genus B., in which are included all species afterwards brought to light, which agree more nearly with B.

than with the types of A. or C. In like manner a new formation is met with in geology, and the characters of its fossil fauna and flora investigated. From that moment it is considered as a record of a certain period of the earth's history, and a standard to which other deposits may be compared. If any are found containing the same or nearly the same organic remains, and occupying the same relative position, they are regarded in the light of contemporary annals. All such monuments are said to relate to one period, during which certain events occurred, such as the formation of particular rocks by aqueous or volcanic agency, or the continued existence and fossilization of certain tribes of animals and plants. When several of these periods have had their true places a.s.signed to them in a chronological series, others are discovered which it becomes necessary to intercalate between those first known; and the difficulty of a.s.signing clear lines of separation must unavoidably increase in proportion as chasms in the past history of the globe are filled up.

Every zoologist and botanist is aware that it is a comparatively easy task to establish genera in departments which have been enriched with only a small number of species, and where there is as yet no tendency in one set of characters to pa.s.s almost insensibly, by a mult.i.tude of connecting links, into another. They also know that the difficulty of cla.s.sification augments, and that the artificial nature of their divisions becomes more apparent, in proportion to the increased number of objects brought to light. But in separating families and genera, they have no other alternative than to avail themselves of such breaks as still remain, or of every hiatus in the chain of animated beings which is not yet filled up. So in geology, we may be eventually compelled to resort to sections of time as arbitrary, and as purely conventional, as those which divide the history of human events into centuries. But in the present state of our knowledge, it is more convenient to use the interruptions which still occur in the regular sequence of geological monuments, as boundary lines between our princ.i.p.al groups or periods, even though the groups thus established are of very unequal value.

The isolated position of distinct tertiary deposits in different parts of Europe has been already alluded to. In addition to the difficulty presented by this want of continuity when we endeavour to settle the chronological relations of these deposits, another arises from the frequent dissimilarity in mineral character of strata of contemporaneous date, such, for example, as those of London and Paris before mentioned.

The ident.i.ty or non-ident.i.ty of species is also a criterion which often fails us. For this we might have been prepared, for we have already seen, that the Mediterranean and Red Sea, although within 70 miles of each other, on each side of the Isthmus of Suez, have each their peculiar fauna; and a marked difference is found in the four groups of testacea now living in the Baltic, English Channel, Black Sea, and Mediterranean, although all these seas have many species in common. In like manner a considerable diversity in the fossils of different tertiary formations, which have been thrown down in distinct seas, estuaries, bays, and lakes, does not always imply a distinctness in the times when they were produced, but may have arisen from climate and conditions of physical geography wholly independent of time. On the other hand, it is now abundantly clear, as the result of geological investigation, that different sets of tertiary strata, immediately superimposed upon each other, contain distinct imbedded species of fossils, in consequence of fluctuations which have been going on in the animate creation, and by which in the course of ages one state of things in the organic world has been subst.i.tuted for another wholly dissimilar.

It has also been shown that in proportion as the age of a tertiary deposit is more modern, so is its fauna more a.n.a.logous to that now in being in the neighbouring seas. It is this law of a nearer agreement of the fossil testacea with the species now living, which may often furnish us with a clue for the chronological arrangement of scattered deposits, where we cannot avail ourselves of any one of the three ordinary chronological tests; namely, superposition, mineral character, and the specific ident.i.ty of the fossils.

Thus, for example, on the African border of the Red Sea, at the height of 40 feet, and sometimes more, above its level, a white calcareous formation has been observed, containing several hundred species of sh.e.l.ls differing from those found in the clay and volcanic tuff of the country round Naples, and of the contiguous island of Ischia. Another deposit has been found at Uddevalla, in Sweden, in which the sh.e.l.ls do not agree with those found near Naples. But although in these three cases there may be scarcely a single sh.e.l.l common to the three different deposits, we do not hesitate to refer them all to one period (the Post-Pliocene), because of the very close agreement of the fossil species in every instance with those now living in the contiguous seas.

To take another example, where the fossil fauna recedes a few steps farther back from our own times. We may compare, first, the beds of loam and clay bordering the Clyde in Scotland (called glacial by some geologists), secondly, others of fluvio-marine origin near Norwich, and, lastly, a third set often rising to considerable heights in Sicily, and we discover that in every case more than three-fourths of the sh.e.l.ls agree with species still living, while the remainder are extinct. Hence we may conclude that all these, greatly diversified as are their organic remains, belong to one and the same era, or to a period immediately antecedent to the Post-Pliocene, because there has been time in each of the areas alluded to for an equal or nearly equal amount of change in the marine testaceous fauna.

Contemporaneousness of origin is inferred in these cases, in spite of the most marked differences of mineral character or organic contents, from a similar degree of divergence in the sh.e.l.ls from those now living in the adjoining seas. The advantage of such a test consists in supplying us with a common point of departure in all countries, however remote.

But the farther we recede from the present times, and the smaller the relative number of recent as compared with extinct species in the tertiary deposits, the less confidence can we place in the exact value of such a test, especially when comparing the strata of very distant regions; for we cannot presume that the rate of former alterations in the animate world, or the continual going out and coming in of species, has been every where exactly equal in equal quant.i.ties of time. The form of the land and sea, and the climate, may have changed more in one region than in another; and consequently there may have been a more rapid destruction and renovation of species in one part of the globe than elsewhere. Considerations of this kind should undoubtedly put us on our guard against relying too implicitly on the accuracy of this test; yet it can never fail to throw great light on the chronological relations of tertiary groups with each other, and with the Post-Pliocene period.

We may derive a conviction of this truth not only from a study of geological monuments of all ages, but also by reflecting on the tendency which prevails in the present state of nature to a uniform rate of simultaneous fluctuation in the flora and fauna of the whole globe. The grounds of such a doctrine cannot be discussed here, and I have explained them at some length in the third Book of the Principles of Geology, where the causes of the successive extinction of species are considered. It will be there seen that each local change in climate and physical geography is attended with the immediate increase of certain species, and the limitation of the range of others. A revolution thus effected is rarely, if ever, confined to a limited s.p.a.ce, or to one geographical province of animals or plants, but affects several other surrounding and contiguous provinces. In each of these, moreover, a.n.a.logous alterations of the stations and habitations of species are simultaneously in progress, reacting in the manner already alluded to on the first province. Hence, long before the geography of any particular district can be essentially altered, the flora and fauna throughout the world will have been materially modified by countless disturbances in the mutual relation of the various members of the organic creation to each other. To a.s.sume that in one large area inhabited exclusively by a single a.s.semblage of species any important revolution in physical geography can be brought about, while other areas remain stationary in regard to the position of land and sea, the height of mountains, and so forth, is a most improbable hypothesis, wholly opposed to what we know of the laws now governing the aqueous and igneous causes. On the other hand, even were this conceivable, the communication of heat and cold between different parts of the atmosphere and ocean is so free and rapid, that the temperature of certain zones cannot be materially raised or lowered without others being immediately affected; and the elevation or diminution in height of an important chain of mountains or the submergence of a wide tract of land would modify the climate even of the antipodes.

It will be observed that in the foregoing allusions to organic remains, the testacea or the sh.e.l.l-bearing mollusca are selected as the most useful and convenient cla.s.s for the purposes of general cla.s.sification. In the first place, they are more universally distributed through strata of every age than any other organic bodies. Those families of fossils which are of rare and casual occurrence are absolutely of no avail in establishing a chronological arrangement. If we have plants alone in one group of strata and the bones of mammalia in another, we can draw no conclusion respecting the affinity or discordance of the organic beings of the two epochs compared; and the same may be said if we have plants and vertebrated animals in one series and only sh.e.l.ls in another. Although corals are more abundant, in a fossil state, than plants, reptiles, or fish, they are still rare when contrasted with sh.e.l.ls, especially in the European tertiary formations. The utility of the testacea is, moreover, enhanced by the circ.u.mstance that some forms are proper to the sea, others to the land, and others to freshwater. Rivers scarcely ever fail to carry down into their deltas some land sh.e.l.ls, together with species which are at once fluviatile and lacustrine. By this means we learn what terrestrial, freshwater, and marine species co-existed at particular eras of the past; and having thus identified strata formed in seas with others which originated contemporaneously in inland lakes, we are then enabled to advance a step farther, and show that certain quadrupeds or aquatic plants, found fossil in lacustrine formations, inhabited the globe at the same period when certain fish, reptiles, and zoophytes lived in the ocean.

Among other characters of the molluscous animals, which render them extremely valuable in settling chronological questions in geology, may be mentioned, first, the wide geographical range of many species; and, secondly, what is probably a consequence of the former, the great duration of species in this cla.s.s, for they appear to have surpa.s.sed in longevity the greater number of the mammalia and fish. Had each species inhabited a very limited s.p.a.ce, it could never, when imbedded in strata, have enabled the geologist to identify deposits at distant points; or had they each lasted but for a brief period, they could have thrown no light on the connection of rocks placed far from each other in the chronological, or, as it is often termed, vertical series.

Many authors have divided the European tertiary strata into three groups--lower, middle, and upper; the lower comprising the oldest formations of Paris and London before-mentioned; the middle those of Bordeaux and Touraine; and the upper all those newer than the middle group.

When engaged in 1828 in preparing my work on the Principles of Geology, I conceived the idea of cla.s.sing the whole series of tertiary strata in four groups, and endeavouring to find characters for each, expressive of their different degrees of affinity to the living fauna. With this view, I obtained information respecting the specific ident.i.ty of many tertiary and recent sh.e.l.ls from several Italian naturalists, and among others from Professors Bonelli, Guidotti, and Costa. Having in 1829 become acquainted with M. Deshayes, of Paris, already well known by his conchological works, I learnt from him that he had arrived, by independent researches, and by the study of a large collection of fossil and recent sh.e.l.ls, at very similar views respecting the arrangement of tertiary formations. At my request he drew up, in a tabular form, lists of all the sh.e.l.ls known to him to occur both in some tertiary formation and in a living state, for the express purpose of ascertaining the proportional number of fossil species identical with the recent which characterized successive groups; and this table, planned by us in common, was published by me in 1833.[110-A] The number of tertiary fossil sh.e.l.ls examined by M. Deshayes was about 3000; and the recent species with which they had been compared about 5000. The result then arrived at was, that in the lower tertiary strata, or those of London and Paris, there were about 3-1/2 per cent. of species identical with recent; in the middle tertiary of the Loire and Gironde about 17 per cent.; and in the upper tertiary or Subapennine beds, from 35 to 50 per cent. In formations still more modern, some of which I had particularly studied in Sicily, where they attain a vast thickness and elevation above the sea, the number of species identical with those now living was believed to be from 90 to 95 per cent. For the sake of clearness and brevity, I proposed to give short technical names to these four groups, or the periods to which they respectively belonged. I called the first or oldest of them Eocene, the second Miocene, the third Older Pliocene, and the last or fourth Newer Pliocene. The first of the above terms, Eocene, is derived from +eos+, eos, _dawn_, and +kainos+, cainos, _recent_, because the fossil sh.e.l.ls of this period contain an extremely small proportion of living species, which may be looked upon as indicating the dawn of the existing state of the testaceous fauna, no recent species having been detected in the older or secondary rocks.

The term Miocene (from +meion+, meion, _less_, and +kainos+, cainos, _recent_) is intended to express a minor proportion of recent species (of testacea), the term Pliocene (from +pleion+, pleion, _more_, and +kainos+, cainos, _recent_) a comparative plurality of the same. It may a.s.sist the memory of students to remind them, that the _Mi_ocene contain a _mi_nor proportion, and _Pl_iocene a comparative _pl_urality of recent species; and that the greater number of recent species always implies the more modern origin of the strata.

It has sometimes been objected to this nomenclature that certain species of infusoria found in the chalk are still existing, and, on the other hand, the Miocene and Older Pliocene deposits often contain the remains of mammalia, reptiles, and fish, exclusively of extinct species. But the reader must bear in mind that the terms Eocene, Miocene, and Pliocene were originally invented with reference purely to conchological data, and in that sense have always been and are still used by me.