(31) 'Monograph of the Eocene Mollusca' (Ibid.) Fred. E. Edwards.

(32) 'Monograph of the Eocene Mollusca' (Ibid.) Searles V. Wood.

(33) 'Monograph of the Crag Mollusca' (Ibid.) Searles V. Wood.

(34) 'Monograph of the Tertiary Entomostraca' (Ibid.) Rupert Jones.

(35) 'Monograph of the Foraminifera of the Crag' (Ibid.) Rupert Jones, Parker, and H. B. Brady.

(36) 'Monograph of the Radiaria of the London Clay' (Ibid.) Edward Forbes.

(37) 'Monograph of the Cetacea of the Red Crag' (Ibid.) Owen.

(38) 'Monograph of the Fossil Reptiles of the London Clay' (Ibid.) Owen and Bell.

(39) "On the Skull of a Dentigerous Bird from the London Clay of Sheppey"--'Quart. Journ. Geol. Soc.,' vol. xxix. Owen.

(40) 'Oss.e.m.e.ns Fossiles.' Cuvier.

(41) 'Fauna Antiqua Sivalensis.' Falconer and Sir Proby Cautley.

(42) 'Palaeontological Memoirs.' Falconer.

(43) 'Animaux Fossiles et Geologie de l'Attique.' Gaudry.

(44) "Princ.i.p.al Characters of the Dinocerata"--'American Journ. of Science and Arts,' vol. xi. Marsh.

(45) 'Princ.i.p.al Characters of the Brontotheridae' (Ibid.) Marsh.

(46) 'Princ.i.p.al Characters of the Tillodontia' (Ibid.) Marsh.

(47) "Extinct Vertebrata of the Eocene of Wyoming"--'Geological Survey of Montana,' &c., 1872. Cope.

(48) "Ancient Fauna of Nebraska"--'Smithsonian Contributions to Knowledge,' vol. vi. Leidy.

(49) 'Manual of Geology.' Dana.

(50) "Palaeontology and Evolution" (Presidential Address to the Geological Society of London, 1870)--'Quart. Journ. Geol.

Soc.,' vol. xxvi. Huxley.'

(51) 'Mineral Conchology.' Sowerby.

(52) 'Description des Coquilles Fossiles,' &c. Deshayes.

(53) 'Description des Coquilles Tertiaires de Belgique.' Nyst.

(54) 'Fossilen Polypen des Wiener Tertiar-beckens.' Reuss.

(55) 'Palaeontologische Studien uber die alteren Tertiar-schichten der Alpen.' Reuss.

(56) 'Land und Suss-wa.s.ser Conchylien der Vorwelt.' Sandberger.

(57) 'Flora Tertiaria Helvetica.' Heer.

(58) 'Flora Fossilis Arctica.' Heer.

(59) 'Recherches sur le Climat et la Vegetation du Pays Tertiaire.' Heer.

(60) 'Fossil Flora of Great Britain.' Lindley and Hutton.

(61) 'Fossil Fruits and Seeds of the London Clay.' Bowerbank.

(62) "Tertiary Leaf-beds of the Isle of Mull"--'Quart. Journ.

Geol. Soc.,' vol. vii. Edward Forbes.

(63) 'The Geology of England and Wales.' Horace B. Woodward.[25]

[Footnote 25: This work--published whilst these sheets were going through the press--gives to the student a detailed view of all the strata of England and Wales, with their various sub-divisions, from the base of the Palaeozoic to the top of the Tertiary.]

CHAPTER XXI.

THE QUATERNARY PERIOD.

THE POST-PLIOCENE PERIOD.

Later than any of the Tertiary formations are various detached and more or less superficial acc.u.mulations, which are generally spoken of as the _Post-Tertiary formations_, in accordance with the nomenclature of Sir Charles Lyell--or as the _Quaternary formations_, in accordance with the general usage of Continental geologists. In all these formations we meet with no _Mollusca_ except such as are now alive--with the partial and very limited exception of some of the oldest deposits of this period, in which a few of the sh.e.l.ls occasionally belong to species not known to be in existence at the present day. Whilst the _Sh.e.l.l-fish_ of the Quaternary deposits are, generally speaking, identical with existing forms, the _Mammals_ are sometimes referable to living, sometimes to extinct species. In accordance with this, the Quaternary formations are divided into two groups: (1) The _Post-Pliocene_, in which the sh.e.l.ls are almost invariably referable to existing species, but some of the _Mammals are extinct_; and (2) the _Recent_, in which _the sh.e.l.ls and the Mammals alike belong to existing species_. The Post-Pliocene deposits are often spoken of as the Pleistocene formations (Gr. _pleistos_, most; _kainos_, new or recent), in allusion to the fact that the great majority of the living beings of this period belong to the species characteristic of the "new" or Recent period.

The _Recent_ deposits, though of the highest possible interest, do not properly concern the palaeontologist strictly so-called, but the zoologist, since they contain the remains of none but existing animals. They are "Pre-historic," but they belong entirely to the existing terrestrial order. The _Post-Pliocene_ deposits, on the other hand, contain the remains of various extinct Mammals; and though Man undoubtedly existed in, at any rate, the later portion of this period, if not throughout the whole of it, they properly form part of the domain of the palaeontologist.

The Post-Pliocene deposits are extremely varied, and very widely distributed; and owing to the mode of their occurrence, the ordinary geological tests of age are in their case but very partially available. The subject of the cla.s.sification of these deposits is therefore an extremely complicated one; and as regards the age of even some of the most important of them, there still exists considerable difference of opinion. For our present purpose, it will be convenient to adopt a cla.s.sification of the Post-Pliocene deposits founded on the relations which they bear in time to the great "Ice-age" or "Glacial period;" though it is not pretended that our present knowledge is sufficient to render such a cla.s.sification more than a provisional one.

In the early Tertiary period, as we have seen, the climate of the northern hemisphere, as shown by the Eocene animals and plants, was very much hotter than it is at present--partaking, indeed, of a sub-tropical character. In the Middle Tertiary or Miocene period, the temperature, though not so high, was still much warmer than that now enjoyed by the northern hemisphere; and we know that the plants of temperate regions at this time flourished within the Arctic circle. In the later Tertiary or Pliocene period, again, there is evidence that the northern hemisphere underwent a further progressive diminution of temperature; though the climate of Europe generally seems at the close of the Tertiary period to have been if anything warmer, or at any rate not colder, than it is at the present day. With the commencement of the Quaternary period, however, this diminution of temperature became more decided; and beginning with a temperate climate, we find the greater portion of the northern hemisphere to become gradually subjected to all the rigours of intense Arctic cold. All the mountainous regions of Northern and Central Europe, of Britain, and of North America, became the nurseries of huge ice-streams, and large areas of the land appear to have been covered with a continuous ice-sheet.

The Arctic conditions of this, the well-known "Glacial period,"

relaxed more than once, and were more than once re-established with lesser intensity. Finally, a gradual but steadily progressive amelioration of temperature took place; the ice slowly gave way, and ultimately disappeared altogether; and the climate once more became temperate, except in high northern lat.i.tudes.

The changes of temperature sketched out above took place slowly and gradually, and occupied the whole of the Post-Pliocene period.

In each of the three periods marked out by these changes--in the early temperate, the central cold, and the later temperate period--certain deposits were laid down over the surface of the northern hemisphere; and these deposits collectively const.i.tute the Post-Pliocene formations. Hence we may conveniently cla.s.sify all the acc.u.mulations of this age under the heads of (1) _Pre-Glacial_ deposits, (2) _Glacial_ deposits, and (3) _Post-Glacial_ deposits, according as they were formed before, during, or after the "Glacial period." It cannot by any means be a.s.serted that we can definitely fix the precise relations in time of all the Post-Pliocene deposits to the Glacial period. On the contrary, there are some which hold a very disputed position as regards this point; and there are others which do not admit of definite allocation in this manner at all, in consequence of their occurrence in regions where no "Glacial Period" is known to have been established.

For our present purpose, however, dealing as we shall have to do princ.i.p.ally with the northern hemisphere, the above cla.s.sification, with all its defects, has greater advantages than any other that has been yet proposed.

I. PRE-GLACIAL DEPOSITS.--The chief pre-glacial deposit of Britain is found on the Norfolk coast, reposing upon the Newer Pliocene (Norwich Crag), and consists of an ancient land-surface which is known as the "Cromer Forest-bed."

This consists of an ancient soil, having embedded in it the stumps of many trees, still in an erect position, with remains of living plants, and the bones of recent and extinct quadrupeds. It is overlaid by fresh-water and marine beds, all the sh.e.l.ls of which belong to existing species, and it is finally surmounted by true "glacial drift." While all the sh.e.l.ls and plants of the Cromer Forest-bed and its a.s.sociated strata belong to existing species, the Mammals are partly living, partly extinct. Thus we find the existing Wolf (_Canis lupus_), Red Deer (_Cervus elaphus_), Roebuck (_Cervus capreolus_), Mole (_Talpa Europtoea_), and Beaver (_Castor fiber_), living in western England side by side with the _Hippopotamus major, Elephas antiquus, Elephas meridionalis, Rhinoceros Etruscus_, and _R. Megarhinus_ of the Pliocene period, which are not only extinct, but imply an at any rate moderately warm climate. Besides the above, the Forest-bed has yielded the remains of several extinct species of Deer, of the great extinct Beaver (_Trogontherium Cuvieri_), of the Caledonian Bull or "Urus"

(_Bos primigenius_), and of a Horse (_Equus fossilis_), little if at all distinguishable from the existing form.

The so-called "Bridlington Crag" of Yorkshire, and the "Chillesford Beds" of Suffolk, are probably to be regarded as also belonging to this period; though many of the sh.e.l.ls which they contain are of an Arctic character, and would indicate that they were deposited in the commencement of the Glacial period itself. Owing, however, to the fact that a few of the sh.e.l.ls of these deposits are not known to occur in a living condition, these, and some other similar acc.u.mulations, are sometimes considered as referable to the Pliocene period.

II. GLACIAL DEPOSITS.--Under this head is included a great series of deposits which are widely spread over both Europe and America, and which were formed at a time when the climate of these countries was very much colder than it is at present, and approached more or less closely to what we see at the present day in the Arctic regions. These deposits are known by the general name of the _Glacial deposits_, or by the more specialised names of the Drift, the Northern Drift, the Boulder-clay, the Till, &c.

These glacial deposits are found in Britain as far south as the Thames, over the whole of Northern Europe, in all the more elevated portions of Southern and Central Europe, and over the whole of North America, as far south as the 39th parallel. They generally occur as sands, clays, and gravels, spread in widely-extended sheets over all the geological formations alike, except the most recent, and are commonly spoken of under the general term of "Glacial drift." They vary much in their exact nature in different districts, but they universally consist of one, or all, of the following members:--

1. _Unstratified_ clays, or loams, containing numerous angular or sub-angular blocks of stone, which have often been transported for a greater or less distance from their parent rock, and which often exhibit polished, grooved, or striated surfaces. These beds are what is called _Boulder-clay_, or _Till_.

2. Sands, gravels, and clays, often more or less regularly _stratified_, but containing erratic blocks, often of large size, and with their edges _unworn_, derived from considerable distances from the place where they are now found. In these beds it is not at all uncommon to find fossil sh.e.l.ls; and these, though of existing species, are generally of an Arctic character, comprising a greater or less number of forms which are now exclusively found in the icy waters of the Arctic seas. These beds are often spoken of as "Stratified Drift."

3. _Stratified_ sands and gravels, in which the pebbles are _worn_ and rounded, and which have been produced by a rearrangement of ordinary glacial beds by the sea. These beds are commonly known as "Drift-gravels," or "Regenerated Drift".

Some of the last-mentioned of these are doubtless post-glacial; but, in the absence of fossils, it is often impossible to arrive at a positive opinion as to the precise age of superficial acc.u.mulations of this nature. It is also the opinion of high authorities that a considerable number of the so-called "cave-deposits," with the bones of extinct Mammals, truly belong to the Glacial period, being formed during warm intervals when the severity of the Arctic cold had become relaxed. It is further believed that some, at any rate, of the so-called "high-level" river-gravels and "brick-earths" have likewise been deposited during mild or warm intervals in the great age of ice; and in two or three instances this has apparently been demonstrated--deposits of this nature, with the bones of extinct animals and the implements of man, having been shown to be overlaid by true Boulder-clay.

The fossils of the undoubted Glacial deposits are princ.i.p.ally sh.e.l.ls, which are found in great numbers in certain localities, sometimes with _Foraminifera_, the bivalved cases of Ostracode Crustaceans, &c. Whilst some of the sh.e.l.ls of the "Drift" are such as now live in the seas of temperate regions, others, as previously remarked, are such as are now only known to live in the seas of high lat.i.tudes; and these therefore afford unquestionable evidence of cold conditions. Amongst these Arctic forms of sh.e.l.ls which characterise the Glacial beds may be mentioned _Pecten Islandicus_ (fig. 254), _Pecten Groenlandicus, Scalaria Groenlandica, Leda truncata, Astarte borealis, Tellina proxima, Nattra clausa_, &c.

[Ill.u.s.tration: Fig. 254.--Left valve of _Pecten Islandicus_, Glacial and Recent.]

III. POST-GLACIAL DEPOSITS.--As the intense cold of the Glacial period became gradually mitigated, and temperate conditions of climate were once more re-established, various deposits were formed in the northern hemisphere, which are found to contain the remains of extinct Mammals, and which, therefore, are clearly of Post-Pliocene age. To these deposits the general name of _Post-Glacial_ formations is given; but it is obvious that, from the nature of the case, and with our present limited knowledge, we cannot draw a rigid line of demarcation between the deposits formed towards the close of the Glacial period, or during warm "interglacial" periods, and those laid down after the ice had fairly disappeared. Indeed it is extremely improbable that any such rigid line of demarcation should ever have existed; and it is far more likely that the Glacial and Post-Glacial periods, and their corresponding deposits, shade into one another by an imperceptible gradation. Accepting this reservation, we may group together, under the general head of "Post-Glacial Deposits,"

most of the so-called "Valley-gravels," "Brick-earths," and "Cave-deposits," together with some "raised beaches" and various deposits of peat. Though not strictly within the compa.s.s of this work, a few words may be said here as to the origin and mode of formation of the Brick-earths, Valley-gravels, and Cave-deposits, as the subject will thus be rendered more clearly intelligible.

Every river produces at the present day beds of fine mud and loam, and acc.u.mulations of gravel, which it deposits at various parts of its course--the gravel generally occupying the lowest position, and the finer sands and mud coming above. Numerous deposits of a similar nature are found in most countries in various localities, and at various heights above the present channels of our rivers. Many of these fluviatile (Lat. _fluvius_, a river) deposits consist of fine loam, worked for brick-making, and known as "Brick-earths;" and they have yielded the remains of numerous extinct Mammals, of which the Mammoth (_Elephas primigenius_) is the most abundant. In the valley of the Rhine these fluviatile loams (known as "Loess") attain a thickness of several hundred feet, and contain land and fresh-water sh.e.l.ls of existing species.

With these occur the remains of Mammals, such as the Mammoth and Woolly Rhinoceros. Many of these Brick-earths are undoubtedly Post-Glacial, but others seem to be clearly "inter-glacial;" and instances have recently been brought forward in which deposits of Brick-earth containing bones and sh.e.l.ls of fresh-water Molluscs have been found to be overlaid by regular unstratified boulder-clay.

The so-called "Valley-gravels," like the Brick-earths, are fluviatile deposits, but are of a coa.r.s.er nature, consisting of sands and gravels. Every river gives origin to deposits of this kind at different points along the course of its valley; and it is not uncommon to find that there exist in the valley of a single river two or more sets of these gravel-beds, formed by the river itself, but formed at times when the river ran at different levels, and therefore formed at different periods. These different acc.u.mulations are known as the "high-level" and "low-level" gravels; and a reference to the accompanying diagram will explain the origin and nature of these deposits (fig. 255). When a river begins to occupy a particular line of drainage, and to form its own channel, it will deposit fluviatile sands and gravels along its sides. As it goes on deepening the bed or valley through which it flows, it will deposit other fluviatile strata at a lower level beside its new bed. In this way have arisen the terms "high-level" and "low-level" gravels. We find, for instance, a modern river flowing through a valley which it has to a great extent or entirely formed itself; by the side of its immediate channel we may find gravels, sand, and loam (fig. 255, 2 2') deposited by the river flowing in its present bed. These are _recent_ fluviatile or alluvial deposits. At some distance from the present bed of the river, and at a higher level, we may find other sands and gravels, quite like the recent ones in character and origin, but formed at a time when the stream flowed at a higher level, and before it had excavated its valley to its present depth. These (fig. 255, 3 3') are the so-called "_low-level_ gravels" of a river. At a still higher level, and still farther removed from the present bed of the river, we may find another terrace, composed of just the same materials as the lower one, but formed at a still earlier period, when the excavation of the valley had proceeded to a much less extent. These (fig. 255, 4 4') are the so-called "_high-level_ gravels" of a river, and there may be one or more terraces of these.

[Ill.u.s.tration: Fig. 255.--Recent and Post-Pliocene Alluvial Deposits.

1, Peat of the recent period; 2, Gravel of the modern river: 2', Loam of the modern river; 3. Lower-level valley-gravel with bones of extinct Mammals (Post-Pliocene); 3', Loam of the same age as 3; 4. Higher-level valley-gravel (Post-Pliocene); 4', Loam of the same age as 4; 5. Upland gravels of various kinds (often glacial drift); 6, Older rock. (After Sir Charles Lyell.)]

The important fact to remember about these fluviatile deposits is this--that here the ordinary geological rule is reversed. The high-level gravels are, of course, the highest, so far as their actual elevation above the sea is concerned; but geologically the lowest, since they are obviously much older than the low-level gravels, as these are than the recent gravels. How much older the high-level gravels may be than the low-level ones, it is impossible to say. They occur at heights varying from 10 to 100 feet above the present river-channels, and they are therefore older than the recent gravels by the time required by the river to dig out its own bed to this depth. How long this period may be, our data do not enable us to determine accurately; but if we are to calculate from the observed rate of erosion of the actually existing rivers, the period between the different valley-gravels must be a very long one.

The lowest or recent fluviatile deposits which occur beside the bed of the present river, are referable to the Recent period, as they contain the remains of none but living Mammals. The two other sets of gravels are Post-Pliocene, as they contain the bones of extinct Mammals, mixed with land and fresh-water sh.e.l.ls of existing species. Among the more important extinct Mammals of the low-level and high-level valley-gravels may be mentioned the _Elephas antiquus_, the Mammoth (_Elephas primigenius_), the Woolly Rhinoceros (_R. Tichorhinus_), the Hippopotamus, the Cave-lion, and the Cave-bear. Along with these are found unquestionable traces of the existence of Man, in the form of rude flint implements of undoubted human workmanship.

The so-called "Cave-deposits," again, though exhibiting peculiarities due to the fact of their occurrence in caverns or fissures in the rocks, are in many respects essentially similar to the older valley-gravels. Caves, in the great majority of instances, occur in limestone. When this is not the case, it will generally be found that they occur along lines of sea-coast, or along lines which can be shown to have anciently formed the coast-line. There are many caves, however, in the making of which it can be shown that the sea has had no hand; and these are most of the caves of limestone districts. These owe their origin to the solvent action upon lime of water holding carbonic acid in solution.