Chap17

[270]

CHAPTER XVII.

PLIOCENE STRATA.

THE PLIOCENE STRATA, or Newer Tertiary beds, form the succeeding division of the Tertiary series, and in England are represented by the various subdivisions of the CRAG of Suffolk and Norfolk. Resting, as these strata generally do, on an eroded surface of the London Clay, or more rarely, on the Chalk, their lower boundary is sufficiently clear; but the same is not the case with the tipper limit of the Crag series, about which diversities of opinion exist, arising, no doubt, from the circumstance that it is absolutely indefinable, there being a kind of passage from the uppermost beds of Crag into overlying strata partly of drifts and glacial detritus. The whole series of Crags and disputed Crags is, indeed, probably not. more than from 120 to 150 feet thick, and they make no decided mark on the physical geography of the country, though important in other points of view. After much consideration I incline, for the present, to restrict the term Crag to the following subdivisions:

The Coralline or White Crag lies on the London Clay in Suffolk, and consists of a few patches found in

[Coralline Crag. 271]

an area of about 20 miles in length, between the River Stour and Aldborough. It is generally not more than 60 feet in thickness. It consists in places almost entirely of Polyzoa (formerly called Corallines, whence the name, Coralline Crag), and elsewhere, in great part, of broken and entire shells, fragments of Echini, &c. Only four genera of Corals are known, all, according to the lists of Mr. Etheridge, of extinct species, and the same authority gives about 140 species of Polyzoa. The genera of Mollusca are almost entirely recent. The general character of the climate seems to have been milder than at present.

According to the researches of Mr. Searles Wood, modified by Mr. Gwyn Jeffreys and Prof. Prestwich, the

FIG. 52.

Group of Coralline Crag Fossils.

Coralline Crag contains 316 species of Mollusca, only 5 of which are Brachiopoda, Argiope cistellula, Ltngula Dumontieri, Orbicula lameliosa, Terebratula grandis, and Terebratulina caput-serpentis. Of the Lamellibranchiata

[272 Coralline and Red Crag Species.]

151 species, of the Gasteropoda 160, and 1 Pteropod, Cleodora infundibulum. Of the 316 species only 52 are said to be extinct, or about 16 per cent. or, in other words, 84 per cent. are still living. Sixteen species of Echinodermata are known, 6 of which still live; and fish are found identical with living species of Cod, Pollack, and Whiting, together with large teeth of a shark, Carcharodom megalodon, Otodus, Raia antiqua, &c. It is quite possible that the Coralline Crag beds may be approximately of the same age with the marine shell beds of the Faluns of Touraine, in France, commonly called Miocene.

The Red Crag is chiefly a ferruginous sand, often crowded with shells entire and broken, very irregularly bedded, in a manner which shows that it was deposited partly in shallow seas, with strong tidal currents near shore, and, indeed, was partly accumulated between the high and low water lines.

At Fellxstow the Red Crag is well seen on the seacliff, lying directly on the London Clay. It is crowded with shells, many of them broken, and was evidently deposited in shallow water. At Walton-on-the-Naze, where it also lies on London Clay, the sea was deeper, the shells being often unbroken, and in the state in which they died.

A hundred-and-forty species are common to the Red and Coralline Crag. In 234 species of shells, 150 now live in British seas, while '32 are now restricted to more southern and 23 to more northern seas' (Prestwich). In all about 92 per cent. of the Mollusca are said to be still living. In 25 species of corals, 14 still inhabit our coasts. Among its characteristic shells are Trophon antiquum (Fusus cntrarius), and various species of Murex, Voluta, Buccinum, Natica, Purpura,

[Red Crag Fossils. 273]

&c. Lamellibranchiate shells are still more common, such as Mactra, Tellina, Cyprina, Astarte, .Pectunculus, Pecten, Cardium, &c. It is not unlikely that

FIG. 53.

Group of Red Crag Fossils.

some of the shells may have been derived from the waste of the Coralline Crag.

At the base of the Coralline Crag, in a pit by the Church near Felixstow, and in other places, there

[274 Fish and Mammalia.]

is frequently a stratum full of phosphatic remains, and known as the Coprolite bed. In it are found many sharks' teeth, vertebræ of fish, and many earbones and occasional vertebræ and other bones of whales. The sharks' teeth have often been derived from the London Clay, and the whales' bones are always very much water-worn, and have altogether a much more ancient appearance than that of the ordinary fossils of the Crag.

Among them are the bones and teeth of land mammalia of extinct species, Castor veterior (beaver), Cervus dicranoceros (deer), Equus plicidens (horse) and Hipparion, Hyœna antiqua and Felis pardoides, Mastodon Arvernensis, M. tapiroides and Elephas Meridionalis, Rhinoceros Schleiermacheri and Sus antiquus. Similar phosphatic remains, though fewer in number, have been found with bones of whales at the base of the Coralline Crag at Sutton. In both cases, many of the bones, &c., are worn and mineralised, and the question is, whether or not the greater part of these terrestrial mammalia belonged to the Crag epoch?

So plentiful are these, that to separate them from the Crag, for the manufacture of manure, forms a profitable branch of commerce.

There are many reasons for believing that during the later part of the Eocene and all through the Miocene epoch, the area now called Britain was joined to the Continent. The physical geography of the country was different, with, however, a general identity in so far that, as already shown, the Palaeozoic mountainous regions now were mountainous then, while between them lay broad plains of secondary formations. In late Miocene times mammalian races must have inhabited

[Norwich crag. 275]

our region, and their bones been scattered on the surface. A partial submergence of the country took place, so that Britain became for a time an island, and the marine Crag beds were deposited over part of our eastern area, the relics of which still remain in Norfolk and Suffolk. Some of the mammalia survived this partial submergence, and continued to inhabit the island during Pliocene times, and getting associated with varieties and new species, the bones of some of the extinct species may have been mingled with others then living, and all were washed into the basement beds of the above-named Crag formations during various oscillations of level.

The Mammaliferous or Norwich Crag consists of sand, gravel, and shells, generally only a few feet in thickness, and which, in Norfolk, lie upon the Chalk. From the nature of the fossils of the Norwich Crag, it is believed to have accumulated near the mouth of a river. It is never seen in contact with or overlying either the Coralline or Red Crag, and it is considered by Mr. Prestwich to be of the same age with the Red Crag,. having been accumulated in an area partly estuarine, and separated from the purely marine area of th Red Crag by an emerged district consisting of the Coralline Crag.

In the Norwich Crag 139 species of marine Mollusca are known, of which 87, or 56 per cent. are common to the Coralline Crag, 137, or 88 per cent. to the Red Crag, and 93 1/2 per cent. are still living. 'Comparing the three Crags the proportions of extinct species of marine Mollusca are, Coralline Crag 16 per cent. Red Crag, 7.7 per cent. and Norwich Crag 6.5 per cent.' (Prestwich). The latter contains about 20 species of land and fresh-water shells, such as Helix, Planorbis, Paludina (P. lenta, &c.), Pupa, Limnœa, Cyclas,

[276 Chillesford Beds.]

Cyrena, &c., most of which are of living species. Besides these, there are found in it the bones of Mastodon Arvernensis, Elephas meridionalis (?), E. antiquus, and Hippopotamus major (?) together with the Horse, Equus fossilis, Castor fiber (beaver), the common Otter, Deer, &c.

The Chillesford Clay and Sand are generally considered to form part of the Norwich Crag series of strata. In the Chillesford district, which is inland, the Clay may lie to some extent unconformably on the Red Crag at Chillesford, and on the Coralline Crag near Sudbourne. In the Norwich district, it is a somewhat inconstant bed, or set of wedge-shaped beds, which, according to Mr. Horace Woodward, occur at different horizons in the Norwich Crag series. It is a very insignificant subformation, as regards thickness, and its marine fossils found in the sands are almost all of living species. The Bure valley beds, characterised by the presence of Tellina Balthica, may possibly form an upper part of the Norwich Crag series. They lie on the well-known Forest bed of Cromer, and together these may connect the uppermost Crag beds with the succeeding Glacial epoch.

It is frequently impossible to identify these minor subdivisions in areas even a small distance apart, for their identity is assumed to rest on the occurrence of certain assemblages of shells, and opinions on some points are so conflicting, that while some geologists consider the Forest bed to be older than all of the Norwich Crag deposits, others maintain that it is newer.

The last great Glacial epoch, Bone caves, River gravels, &c., will be treated of in succeeding chapters. These, less or more, belong to the age of human history,

[History of Geological Opinion. 277]

and are thus intimately related to the physical geography of the time in which we live.

Thus ends my brief sketch of the geographical range of the geological formations of Britain, in which, for obvious reasons, I have largely directed the attention of the reader to the snbject of the Physical Geography of the British area during the epochs in which they were deposited.

It took a long time, by analyses of the order of deposition of stratified rocks and their contents, for geologists to establish the facts and reasonings now generally accepted, and the chief advances have been made in the last eighty years, beginning with the work of Hutton and William Smith. Notices occur in the pages of Herodotus, Aristotle, Strabo, and Pliny, which scarcely amount to geological ideas, but which show that they were cognisant of the occurrence of shells far inland, and high on the mountains; and they also reasoned on the mutability of the relative levels and positions of sea and land.

In the fifth century, Orosius, a Spanish divine, recognised the true nature of fossil shells, but referred them to the Deluge; and this opinion for long prevailed among such men as Lister (1683), Burnet (1690), Woodward (1695), and many more besides. Others in Italy (Olivi, 1584, Scilla, 1670, &c.), France, and England (Dr. Plot, 167 7), held the absurd opinion that they were 'sports of nature,' the result of the fermentation of a 'materia pinguis, or fatty matter'; or that 'petrified shells were stones in disguise, formed by the influence of the heavenly bodies.' A few remarkable men held more correct views on the subject. In 1580, 'a potter,' says Fontenelle, 'who knew neither Latin

[278 Palissy, Mitchell, and Fuchsel.]

nor Greek, was the first who dared assert in Paris, and to the face of all the doctors, that fossil shells were true shells, deposited formerly in the sea in the places where they are found, . . . and he stoutly defied all the school of Aristotle to attack his proofs. This man was Bernard Palissy, Saintongeois, as great a physician as unassisted nature can produce.' In 1669, Steno published his remarkable treatise De Solido intra Solidum naturaliter Contento, in which he demonstrated that plants, shells, and teeth found in rocks are truly organic; and that they were buried in marine sediments, in the same manner that the remains of plants and marine animals are now entombed in modern sea bottoms. Hook, in his Discourse of Earthquakes (1688), maintains like opinions; and he inferred the extinction of species, and the introduction of varieties, consequent on changes in physical geography. Still further, he speaks of the 'records of antiquity which nature has left as monuments and hieroglyphic characters of preceding transactions; . . . and though it is very difficult to read them, and to raise a chronology out of them, . . . yet 'tis not impossible.' This is the earliest distinct hint of the principle of succession of life in time.

In 1760, Mitchell, in his Memoir on Earthquakes, in the 'Philosophical Transactions,' shows a clear perception of an order of superposition in strata, but he does not combine it with the fact of a parallel succession of life. About this time matters begin to become more definite, and a physician of Rudelstadt, George Christian Fuchsel, showed a remarkable knowledge both of the succession of stratified formations and of the succession of life in time, and his writings contain even more than the germ of many of the truths that, during the present

[Werner and Hutton. 279]

century, have given so rapid an impulse to the science. In his system he distinguishes, resting on primitive schists, thirteen subformations, which in modern phrase, range between the rothe todte¹ and the Muschelkalk, each being characterised by a distinctive assemblage of fossils, or peculiarities of marine and fluviatile deposition, the carbonaceous strata being attributed to exotic plants of marshes and forests, the accumulation of which, by means of river floods, has produced coal.² As these formations contain remains of land plants and animals, the seas in which the strata were formed must have surrounded an ancient continent, which at an earlier date was also formed of strata after the manner of those he described, and this again by a continent older still, for he taught that the physical phenomena of the earth are constant and unchangeable.

Rather later, Werner, by his enthusiasm, eloquence, and skill as a mineralogist, also lent some aid to the cause; but his ignorant and bigoted adherence to the dogma that all rocks are aqueous, did much to retard the advance of truth. His far greater opponent, Hutton (1788), in his Theory of the Earth, expounded the true doctrine, which may be summed up as follows:—
    1st. That, in the known geological history of the world, the course of events has never been disturbed by universal paroxysmal catastrophes, but that the course of change has been similar to that of the existing economy of nature.

¹ The passage is a little obscure: the words rothe todte would seem to imply that the strata are of Permian age, while the statement that the strata lie beneath the formation houillère, would bespeak strata perhaps equivalent to our Old Red Sandstone.

² 'Journal de Geologie,' 1830, p. 192.

[280 Hutton.]

    2nd. That we know of no set of igneous rocks that can be proved to be of generally older origin than the earliest stratified deposits, but that they may often be proved to be of posterior origin.

    3rd. That the stratified masses were formed from the waste of pre-existing rocks, mingled with organic exuviœ.

    4th. That such strata afford a measure of the amount of pre-existing land destroyed to afford materials for their formation.

    5th. That there may be a progressive formation of rocks in the bottom of the sea, contemporaneous with great and repeated alterations of lower strata, that approach the regions of internal heat (metamorphism).

    6th. That all strata being derivative, and a machinery existing capable alike of erecting and destroying rocks, in the whole course of visible nature 'we find no vestige of a beginning-no trace of an end.¹

¹ In these modern days very few persons read Hutton, and those who trouble themselves about old geology are in general more familiar with Playfair's delightful 'Illustrations of the Huttonian Theory' than with Hutton's great original work, in which the philosophy of igneous, stratigraphical, and metamorphic geology was described in a manner that excited the admiring wonder of a few who in those days were able to appreciate his generalisations. One of these was the celebrated Dr. Black, Professor of Chemistry in the University of Glasgow, who thus wrote to the Princess Daschkow. 'In this system of Dr. Hutton there is a grandeur and sublimity by which it far surpasses any that has been offered. The boundless pre-existence of time and the operations of nature which he brings into our view, the depth and extent to which his imagination has explored the action of fire in the internal parts of the earth, strike us with astonishment. And when we consider the view he gives us of a great river, such as that of the Amazons, descending in a thousand streams from the country of the Andes, and forming those immense and level plains through which it flows in a great part of its course, the mind is expanded in contemplating

[William Smith. 281]

To the less precise generalisations of Fuebsel, William Smith added the complete proof of the succession of life in time, proving, as he did, in England, a clear succession of strata, each more or less characterised by its own suite of fossils; and this gave, to a great extent, a perfect clue to the reading of that chronology on which Hook so vaguely speculated. To effect his object, Smith traced the English formations from end to end of the country with unwearied devotion, and at length, in 1815, produced his great geological map of England. He struggled long, almost unrecognised in his labours, but when they were well nigh at an end, men began by degrees to recognise that a master was among them, and in 1831, the first Wollaston medal was awarded to him by the council of the Geological Society, while in his annual address, Professor Sedgwick hailed William Smith as 'the father of English geology.' He died in 1839, and surely his name will last as that of a great original observer, even though it may be surmised that the time was ripe for that discovery, which, unknown to Smith, had been partly forestalled by Fuchsel.

The doctrines of Hutton and Smith combined gave the key to great part of the modern system of geology, and later works have carried out and improved upon their conclusions, in a series of masterly investigations

so great an idea, and the length of time which the change thus imagined (I may say demonstrated) must have required.' I am indebted to Professor Young of Glasgow for this extract from a manuscript volume of Dr. Black's letters preserved in the Hunterian Museum. Dr. Black was born in France in 1728. In 1756 he was appointed Professor of Chemistry in Glasgow, and in 1766 he was transferred to fill the Chair of Chemistry in the University of Edinburgh. He died, at the age of 71, in 1799. He was therefore the contemporary of Hutton ,who died in 1796, and they were intimate friends.

[282 Modern Geology.]

that now entitles geology fairly to take its place among the exact sciences. Few persons now study the old prophets and fathers in geology, and therefore I have thought it well to give the foregoing imperfect sketch of the slow progress of the steps by which at length men have become able to analyse the order of deposition of formations, and of their fossilised contents, as abridged in the foregoing chapters.¹

¹ The words formation, epoch, series, period, are in this book only used as convenient terms. When analysed they often imply that certain links, chapters, or whole books are missing in geological history, epochs in fact unrepresented in given areas by stratified formations. If I were to write a complete history of the British rocks, I would endeavour to explain the special meaning of each of these unrepresented gaps in time. A thorough-going physical geologist, working in concert with a thorough paheontologist, might even hope to form a fair notion of the nature of the missing life of the unrepresented epochs.