Chap22

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CHAPTER XXII.

THE MIOCENE AND PLIOCENE FORMATIONS.

THE Eocene strata of England taken as a whole may be looked upon as estuarine beds. At the base, the Woolwich and Reading beds, and also the upper parts of the series in the Isle of Wight and Hampshire, consist of strata deposited in brackish, salt, and fresh water, at or near the mouth of a great river, and the abundance of plants and terrestrial remains in the London Clay, and other marine divisions of the series, proves that they also were deposited near the mouths of such rivers, say, as the Mississippi and Amazon. Both in their lower and upper divisions, these strata in France and England contain a large terrestrial mammalian fauna, the genera of which are so antique that they have no very close relation with those now living. Nevertheless, they are remotely related to living genera, and some may even be the direct ancestors of living species through Miocene and Pliocene intermediate forms. To give an idea of the antiquity of this old fauna, it is safe to say that when they lived the Alps had scarcely any place as a principal mountain range.

This book has little to do with palæonto1ogy, but I have already stated in a previous chapter, that in Germany there are formations containing terrestrial (as distinguished from marine fossils), with mixed Eocene and Miocene generic forms, and I lay a little

[Miocene Epoch. 353]

stress on these points, because, after we get through these doubtful and fragmentary stratigraphical and zoological gradations, we at length emerge on a time generally recognised as Miocene or Middle Tertiary, the larger part of the flora and fauna of which has the closest analogy to those that now inhabit the earth, the flora, possibly, even in part, specifically, and part of the fauna, certainly generically. Most of the modern types are represented in one part of the world or another: Elephant, Rhinoceros, Hippopotamus, Horses, Deer, Oxen, Camels, Giraffes, Monkeys, and various carnivora. Nor are fresh-water reptilia wanting, though they are less distinctive, some of the modern representatives of these animals having held their place through longer epochs of time.

I recapitulate these facts, because the circumstances, bearing as they do on the present physical geography of our part of the world, are very distinct, and I shall soon have something more to say about the later unions of England with the Continent, and migrations of life consequent thereon.

The Hempstead beds of the Isle of Wight partly connect the Eocene and Miocene epochs, in so far that the plants of these strata (always an imperfect guide) are related to Miocene species. But stratigraphically, the Hempstead beds are inseparable from the Eocene beds below, and their fossils, those that lived in water, are almost without exception the same.

True Miocene strata are very poorly represented in England, as shown in Chapter XVI., in the description of Bovey Tracey, and they play no important part in its physical geography. The slopes which surrounded the old lake of Bovey Tracey were clothed with splendid pines of the genus Sequoia (Wellingtonia), oaks,

[354 Miocene.]

cinamons, figs, dryandra, prickly vines, nyssa, and other plants, and on the lake water-lilies expanded their leaves and flowers.

The present Europe, partly then a continent, was, in Miocene times, the theatre of wide-spread volcanic eruptions in Central France, Germany, and that part of the British Islands now known as the Inner Hebrides, and also in the north-east of Ireland. In that region they play a much more important part in connection with the physical geography of our country than they do at Bovey Tracey. In the land of Antrim, from thence through the Isles of Mull, Rum, Eigg, Cana, Muck, and Skye, a vast broken belt of Miocene volcanic rocks forms great part of the Inner Hebrides; and far beyond Britain, in the Faroe Islands, Iceland, Spitzbergen, and Franz Joseph Land, the same volcanic series is found, fragments perhaps of one large continuous territory, or, if not, at all events of a series of large islands, of which the Faroes make one of the fragments.

In Scotland these volcanic rocks consist chiefly of Basalts, Dolerites, Feistones, and Amygdaloids, interbedded with agglomerates of lapilli, large blocks, and volcanic bombs, such as are piled on the sides of almost all volcanoes, and which often go by the name of volcanic ashes. These occur, associated with beds of clay or soil, and streaks of lignite. Some of the clays contain leaves of plants. The late Professor Edward Forbes determined their age, and later observations made by Professor Heer confirm his accttracy.¹

¹ For an elaborate memoir on these volcanic rocks, see Mr. Judd 'On the Ancient Volcanoes of the Highlands, &c., 'Journal of the Geological Society,' 1874, vol. xxx., p. 220. The whole subject of the growth and decay of these volcanoes is there treated of in a manner never before attempted.

[Volcanic Rocks. 355]

It is clear that the beds of lignite in the Western Isles, and the shales with leaves, indicate long pauses here and there in the activity of many craters. Vegetation on a large scale had time to flourish. After an unknown lapse of time, the vast inclined plateaux of lava, above which the lofty craters rose, are still, in Antrim, from 600 to 900 feet thick, and more than 3,000 feet in Mull. The denuded edges of the several lava-streams now form a wonderful series of terraces, rising tier upon tier, like Titanic steps, high on the hills on both sides of the Sound of Mull, and the splendid columnar basalt of Fingal's Cave, in Staffa, is known to all tourists among the Inner Hebrides. The same terraced forms are prominent in Skye, and in many of the smaller islands. But where are the craters from which these vast volcanic piles of lavas and ashes were ejected? They are all gone and utterly wasted away, and only their deepseated roots remain to mark the sites, above which mountains grew by accretion, as high as Etna, which is growing even now. It is a remarkable circumstance, and worthy to be noted, that these deep-seated centres of crystalline rocks are now apt to form some of the highest portions of the islands. They have been bared by denudation, and their hardness helps to preserve them.

Long before these extreme denudations took place, when the islands formed part of a widespreading territory, old river-beds intersected it, running through an ancient land, formed of Laurentian, Cambrian, and Silurian rocks, that spread far to the west, north, and east.

These rivers scooped out valleys in the Miocene lavas and tufas, which were again partly filled by torrents of basalt and obsidian. In the case of the

[356 Miocene Denudations.]

Scuir of Eigg, the lava (obsidian 3, fig. 77) flowed over the valley gravel (2) and buried it. Since then the waste has been so great, that the destruction of the older hills that bounded the valley has literally exalted the valley and laid the hills low, for the obsidian was harder than the walls of the valley, and has longer resisted destruction,

FIG. 77.

1. Old lavas, &c. 2. River valley scooped out by denudation. 3. Obsidian filling valley. 4. Present outline of the country produced by later denudations.

and thus it happened that the Scuir of Eigg became one of the most striking peaks in the Western Islands.

Thus it also happens that in the old volcanic plateaux valleys a thousand feet deep have been excavated, and the whole region has by denudation been changed into a line of fragmentary islands, the high sea cliffs of which attest the greatness of the waste they have in time undergone.¹

During this time the greater contours of England remained the same, that is to say, the old mountain lands of Devon, Wales, and Cumbria, though suffering from denudation, remained mountainous still, and, together with the lower country, the whole was merely upheaved so far that England was joined to the

¹ See A. Geikie, 'Jour. Geol. Society,' xxvii., 'On Tertiary Volcanic Rocks.'

[Crag. 357]

Continent, and over the land, mammalian races in late Miocene times migrated into our region, their bones being now found buried at the bases both of the Coralline and Red Crag, but chiefly in the latter. Probably they lived here in the earliest Pliocene times, as the relics of an older Miocene fauna, and got intermixed with varieties and new species. These include Beaver, Deer, Horse and Hipparion, Hyna, and a Felis; Bears, Pig, Tapir, Rhinoceros, Mastodon, and perhaps a true Elephant,¹ all belonging to genera with which we are quite familiar in the present world, if we except the Hipparion and Mastodon, and these have close relations, the first with the horse and the second with the elephant.

The Crag formations of England in descending order consist of three divisions, Norwich Crag, Red Crag, and Coralline Crag. The Red and Coralline Crags are rich in marine fossils, and the Norwich Crag also contains a marine fauna, together with twenty-four species of land and freshwater shells. According to Mr. Prestwich, the above-named formations contain from 84 to 93 per cent. of living species. But though very important in a stratigraphical point of view, when viewed in connection with marine life, the Crag plays a very unimportant part in the physical structure of England, occurring as they do only in a few small shelly patches of insignificant thickness in Norfolk and

¹ Castor veterior, Cervus dicranoceros, Equus plicidens (?), Felis pardoides, Hipparion, Hyena antiqua, Mastodon arvernensis, Mastodon tapiroides (?), Elephas meridionalis (?) Rhinoceros Schleirmacheri, Sus antiquus (?), Tapirus priscus, Ursus arvernensis, Megaceros Hibernicus (?). See Prestwich, 'Journal Geol. Society,' 1871, vol. xxvii., p. 348. Mr. Prestwich considers this fauna as probably of Pliocene age, that is to say, contemporaneous with the deposition of the crag.

[358 Forest Bed.]

Suffolk. They are, in fact, often so far buried under superficial strata that they require to be looked for, and the whole country being flat they do not at all affect the scenery, excepting in a minor way in the coast cliffs. Physically they chiefly indicate a certain amount of submergence and subsequent emergence in late times, before the epoch of the Forest bed, and that is all, for, as already frequently insisted on, we are not to consider Great Britain as having always been an island during and between the periods that I have already described. It is an accident that it is now an island; and it has been islands many times, and an island more than once before, and in many shapes. When I describe other periods, still later than the Crag, we shall be able to understand a little more definitely the precise kind of changes that our land in latter days has undergone.

Younger than the Crag there are certain other minor deposits, portions of which are scattered here and there throughout England. One of the most remarkable, the 'Forest bed,' lies underneath the glacial deposits on the shore, at Cromer, in Norfolk. This minor formation has been traced for some distance between high arid low water mark. It consists of dark sandy clay, with plants, above which there is a band of coarse gravel, containing the remains of elephants, &c., then bands of clay and gravel, with marine and freshwater shells and fragments of wood. The plants noticed in the Forest bed are : Pinus sylvestris (Scotch fir), Abies excelsa (a Pine), Taxus baccata (Yew), Prunus spinosa (Sloe), Menyanthes trifoliata (Buckbean), Quercus (Oak), Alnus (Alder), Nymphœa alba (Water-lily), Nuphar lutea (Yellow Water-lily), Ceratophyllum demersum (Horn-wort), and Potamogeton

[Flora and Fauna. 359]

(Pondweed), together with fronds and rhizomes of ferns.

In the Forest bed and the overlying gravel the following land mammalia have been found: Elephas antiquus (the ancestor of the African Elephant), E. meridiomalis, Rhinoceros megarrhinus, R. Etruscus, Hippopotamus major, Equus caballus (the common horse), Machairodus (a tiger ?), Bison priscus (?), Bos primigenius (Aurochs), Sus Arvernensis; four species of bears, Ursus Arvernensis, U. Spelœus (Cave bear ?), U. Etruscus, U. arctos (White bear) ; six species of deer, Cervus megaceros (often miscalled the Irish elk), C. elaphus (Red deer), C. Sedgwickii, C. Poligniacus, C. capreolus (Roedeer), Mygale moschata (Musk shrew), Sorex fodiens and S. remifer (Shrews), Arvicola amphibia (Field-mouse), Castor Europœus (common beaver), Trogontherium Cuvieri (a great Beaver), two species of whales, and fish.¹ The whole speaks of a past physical geography, at least during part of which, with a mild climate, our country seems to have been joined to the Continent. It must, however, be confessed that this assemblage of mammalia is not quite devoid of the appearance of being a little too miscellaneous, and several authors have declared that some of the bones, having been picked up on the shore between high and low water mark, may have been washed up from the neighbouring sea-bottom, and thus got mixed with others of later geological date which really belong to the Forest bed. However this may be I have given the list as it originally stood, with some slight corrections by Professor Boyd Dawkins, and whichever theory be

¹ The above list is taken from Mr. Prestwich 'On the Crag Beds of Sufolk and Norfolk;' 'Quarter'y Journal of the Geological Society,' vol. xxvii., p. 466.

[360 Forest Bed.]

true, it cannot fail to impress the reader as throwing light on some one of the ever-shifting physical and biological phases that our country has undergone, each of which in its day seemed as constant as that in the midst of which we live. The special episode of the Forest bed points to this, that it exhibits a fragment of the vegetation and fauna of the last pre-glacial epoch, at a time when England was united to the Continent, and when a flora and fauna, in part new, migrated across the intervening plain into our area.