Chap16

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CHAPTER XVI.

MIOCENE EPOCHS

THE MIOCENE STRATA in England play a very unimportant part, in a physical point of view, excepting that the remains of many land plants which they contain give a high interest to these deposits from the light they throw on the climate of the time.

These strata lie not far from the mass of granite that forms the high ground of Dartmoor Forest, the highest point of which, called Yes Tor, is about 2,050 feet above the sea. At the foot of this mountain land there is a plain, of a kind of pear-shaped form, stretching, about seven and a half miles in length, between the neighbourhood of Bovey Tracey and Aller Mills, near Newton Abbot. It is about three miles in its greatest width from Blackpool to Knighton, two miles south-west of Chudleigh.¹

¹ For these and some other details I am indebted to Mr. Horace B. Woodward, of the Geological Survey, who lately re-mapped the Bovey Tracey district, and also to the joint work of Mr. William Pengelly F.R.S. and the well known botanist, Professor Oswald Heer of Zurich, whose masterly description of the plants of the Bovey beds threw the first clear light on the geological age of the strata See Woodward's 'Geology of England and Wales,' and 'On the Lignite Formation of Bovey Tracey,' by Messrs Pengelly and Heer, 'Philosophical Transactions,' 1862. The expense of the production and republication of this work was defrayed by the Baroness Burdett Coutts, who thereby conferred a boon on all students of Tertiary Geology.

[260 Miocene Strata.]

The surface of the plain, according to Mr. Pengelly, consists of sandy clay, which contains a large number of angular and subangular stones lying unconformably on the Miocene strata, which consist of numerous beds of sand, clay, and, in the northern part, of lignite. According to Mr. Horace Woodward, the total thickness of these strata may be from 200 to 300 feet. The whole of these Miocene beds give the impression that they were originally deposited in a lake hollow, the sands and clays having been derived from the waste of the neighbouring Greensand and the granite of Dartmoor, while the vegetable matter that now forms the ligriites consisted of stems and leaves of trees, fruits, ferns, &c., which were drifted by the streams of the time into the lake, where they got water-logged and sank, to be buried in the gradually accumulating strata.

In the northern part of the area, where the Bovey coal (lignite) occurs, near Bovey Tracey, the beds, according to Mr. Pengelly, dip at an angle of 12 1/2°, about 15° south of west, while according to Mr. Woodward, further south they dip much in the same direction about 5°. The lignite division of the strata is separated from the more southern clayey part of the area by a fault, probably of about 100 feet. In the opinion of Mr. Pengelly and Mr. Woodward, the strata south and east of the fault belong to an upper part of the series, which originally spread over that part of the strata in which the beds of lignite are found, having since been removed by denudation.

When we consider the effect of the fault, and also of the inclination of the strata, it is evident that the formation as originally deposited, must have spread beyond its present limits in the direction of the surrounding hills, and that the old lake probably washed

[Bovey Tracey. 261]

the base of the granite hills of Dartmoor on one side, and the slopes of Greensand on the other. Of this there can be little doubt, that the fine clays, often light-coloured, which form so much of the series, were derived from the disintegration of the felspar of the granite of Dartmoor, and the sands, where pure, were probably partly made from the quartz of that granite, and partly from the waste of the neighbouring Upper Greensand hills, then no doubt more extensive and higher than now.

The climate of the period is unmistakably indicated by the plants, of which fifty species have been described by Heer, from these Lower Miocene beds. Some of these are ferns, including Lastrœa stiriaca, and L. Bunburii, Pecopteris lignitum and P. Hookeri, and of the Order Coniferæ we have the branches, fruits, and seeds of the lofty Sequoia Couttsiœ, a genus that abounded in the Miocene epoch, and probably formed the most common tree in the woods that surrounded this lake. The nearest living analogue of this tree is the Sequoia gigantea of California, the Wellingtonia of our parks and gardens. Various grasses have been found, and fragments of a Palm-tree, Palmacites Dœmonorops, probably resembling the living Rotang-palm, and leaves of Oaks, Quercus Lyellii, of which, says Heer, 'similar species are still living in Mexico.' Three species of Figs and two of Grapes have been described, together with Laurels, Cinnamons, Birches, Willows, Waterlilies, &c., all of extinct species according to Heer, and eleven of which are common to the Miocene flora of Switzerland, both 'manifesting a subtropical climate.'

'If,' says Professor Heer, 'from the relics of the Bovey plants, we attempt to represent the vegetation of

[262 Miocene, Bovey Tracey.]

Bovey as it existed in the Tertiary period, we shall have to sketch it somewhat in the following manner:—The woods which covered the slopes which surrounded the beds of lignite, consisted mainly of a huge coniferous tree (Sequoia Couttsiœ), whose figure resembled in all probability its highly admired cousin, the Sequoia (Wellingtonia) gigantea of California. It had just the same graceful slender form in its vernal shoots, thickly studded with leaflets; and the similarity continued in the older shoots and branches, which were clothed with scales. But it presented a distinct character in its shorter leaves, which were even more closely appressed to the shoots, and in its smaller cones. The leafy trees of most frequent occurrence were the cinnamons (Cinnamomum lanceolatum and C. Scheutchzeri) and an evergreen oak (Quercus Lyelii) like those which now are seen in Mexico. The species of evergreen figs were rarer, as were also those of Anona and Gardenia. The trees of the ancient forest were evidently festooned with vines, beside which the prickly Rotang-palm (Palmacites Dœmonorops) twined its snakelike form, In the shade of the forest throve numerous ferns, one species of which, Pecopteris lignitum, seems to have formed trees of imposing grandeur; besides which there were masses of underwood belonging to various species of the genus Nyssa, which is at present confined to North America. On the surface of the lake, in which were formed the deposits of clay and sand that lie between the lignite beds, were expanded the leaves of those water-lilies, the ornate seeds of which are preserved for our examination.'

A description so vivid needs no comment, and of this we may be sure, that this fragment of a flora only represents a small part of that of a vast continent, to

[Antrim and Scotland. 263]

which the British Islands were united, and which, embracing Iceland, spread far to the north and west into the area of what is now the Atlantic, arid on the south was united to Africa, when as yet the Mediterranean had no existence.

In those days our British mountain lands formed of palæozoic rocks were mountainous then as they are now, but higher; and elsewhere, especially after the close of the formation of the Eocene strata, the Alps, the Carpathians, and the Pyrenees, first rose into prominence as mountain chains, at the foot of which in Switzerland were great lakes, from the collective strata of which Professor Heer has numbered 900 species of plants and nearly as many insects, all such as must have lived in a subtropical climate, probably warmer than that of our Devonshire area, if we may judge by the fossilised remains of date-palms.

When, however, we travel northward from Bovey Tracey, the case is different, and to make this plain, I must lead you for a moment through the Western Isles of Scotland, and far beyond, among the islands of the Arctic Sea.

In Antrim, the island of Mull, and on the mainland opposite, and in Staffa, Rum, Eigg, Canna, and Skye, the Miocene rocks consist chiefly of the lava-flows and ashes of great terrestrial volcanoes. These, as they accumulated, overflowed and filled up the undulating valleys of chalk in Antrim, of Oolite and of Silurian gneiss in what is flow the west of Scotland, and in the intervals of eruptions, lakes were sometimes formed, arid terrestrial soils accumulated on the sides of volcanoes, some of which, according to Mr. Judd, grew by accretion of volcanic matter till they rivalled Etna in height., and seemed as if they might last for ever, but

[264 Miocene of Mull and Arctic Regions.]

being diminished by central subsidence and long-continued sub-aerial waste, the mountain of Beinn More in Mull is now only 3,172 feet in height.

No shells of any kind have yet been found in the Bovey beds, nor have any been seen in the Hebrides, but in Mull, at the headland of Ardtum, the Duke of Argyll discovered in 1851 three thin leaf-bearing beds of shale, intercalated among beds of basaltic lava, and tufas or volcanic ashes. These vegetable remains consist of a conifer, Sequoia Langsdorfii, together with Corylus Mac Quarrii, a plane tree, Platanus aceroides, and a fern, Filicites hebridica, as yet only found in Mull. These, and I believe also the Flora of Antrim, are partly considered to belong to a more northern type of vegetation than the plants of Bovey Tracey, and two of the species, the Coryllus and Platanus, are also found in Iceland.

Associated with the volcanic rocks of Skye and the Faröe Islands, similar phenomena occur, with an analogous but still more northern flora, and the early volcanic eruptions of Iceland date back to the Miocene period. There, in beds of lignite called Surturbrand, are found the remains of Pines, Poplars, Elms, Planetrees, Maples, Oaks, Tulip-trees, and Vines, in latitudes where nothing larger than dwarf-birches now grows. Only two of the Iceland species, as stated above, occur in Britain, and even the genera are distinct from those of Bovey, with the exception of Sequoia and an Oak. In Spitzbergen a similar flora of Miocene age occurs, and also in Greenland, far north of the Arctic Circle.

It may seem remarkable that, within the broad area of the British Islands, no mammalian remains have been found in the Miocene strata, for surely a land covered with a wealth of trees, grasses, and other plants could

[Miocene Mammalia. 265]

not have been destitute of animal life of many kinds, both in the waters and on the lands. But on reflection it is not to be wondered at that such remains are absent. In the first place there are no great river deposits of Miocene age remaining in Britain, in which such kinds of organic remains might lie buried, and the only lacusrine strata preserved lie in a small area of a few miles in length at Bovey Tracey. Neither is it likely that bones of mammalia should be found in the thin local soils, of a few inches in thickness, that were formed during the intervals of eruptions on the sides of lofty volcanoes. If, as I believe, a populous mammalian fauna lived and died and left their bones on the land of that old area, these bones decayed, unburied in sediments, and helped to nourish the luxuriant vegetation. But on the adjacent land, of what is now the Continent of Europe, there is no lack of mammalian and other bones to tell us what may have been the kinds of animals that inhabited our now insular area, for in the shallow near-shore deposits of the Faluns of Touraine, we find the Dinotherium, the elephantine Mastodon, Rhinoceros, Hippopotamus, Dichobune, and Deer, and in the fresh-water Miocene strata of the banks of the Rhine, between Bingen and Basle, a similar assemblage occurs.

In Switzerland, between the Alps and the Jura, besides fresh-water shells and spiders, and all the tribes of insects of familiar genera, we find Salamanders, Frogs and Toads, Lizards, Crocodiles, Serpents, Tortoises, and Birds. Of the mammalia in the Swiss strata, thirty-eight genera and fifty-nine species are named by Heer, which approach more closely to the Eocene fauna than to that of the present day. Of these, twenty-nine are extinct, and of the remainder only

[266 Miocene Mammalia and]

Deer, Wild boar, and Squirrels still occur in Switzerland. Of the others, the Lagomys, a hare-like rodent, inhabits the temperate zone in Asia and North America, while five are denizens of warm and torrid zones, viz. the Gibbons in India, the Opossums in South America, the Rhinoceros and Musk-deer in India and Africa, and the Tapirs in India and South America. We may, therefore, believe that the climate was warm. Some of the details of this very interesting fauna are as follows :

In the Miocene rocks of Switzerland there is found an Opossum, Didelphys Blainvillei, Palœotherium Schinzii, a Tapir T. Helveticus, and Listriodon splendens, closely allied to the Tapirs. Also two species of Mastodon, M. tapiroides and M. turicensis, nearly allied to Elephants, and Dinotherium giganteum, somewhat allied to the Mastodon. Five species of Rhinoceros are known besides Anchitherium Aurelianense, a mammal of equine affinities, and Hipparion gracile, which may be described as a three-toed horse, having on each side of the middle hoof two smaller toes which did not touch the ground. Six generas and eleven species of swine-like animals are known, two of which belong to the existing genus Sus, while the others are of extinct genera, but some of them closely allied to those still living. Of ruminant animals we find the Chalicotheriurn antiquum, allied to the Eocene Anoplotheria, and as large as the Indian Rhinoceros, together with two species of ruminants, smaller than a rabbit, Microtherium Renygeri, and M. Cartieri. Besides these there are found fossil, two species of Musk-deer, Moschus Aurelianensis, and M. aquaticus, and Dorcatherium Naui, somewhat allied, but differing in having seven molar teeth in the lower jaw, while the

[Physical Geography. 267]

Musk-deer has only six. Of true Deer there are seven species. Of Rodentia, there are Squirrels, Hares, Chinchillas, and Beavers, and the hare-like rodents Lagomys œningensis and L. Meyeri. Six species of Carnivora have been found of the genera Hyœnodon, Amphicyon, Potamotherium, Trochictis, and Galecynus. The second of these is related to the Dog, the third to the Otter, the fourth to the Weasel and Badger, and the last unites some of the characters of the Dog and the Civet cat.

In the Swiss Miocene series the upper jaw of an Ape was found in the lignite of Elgg, and named Hylobates antiquus, by Lartet. It is most nearly related to the Indian Gibbon. Besides this, two other Apes are known in ground not far off, at Sausan and on the Swabian Alp. They have been named Dryopithecus Fontani, and Semnopithecus pentalicus, the former of which equalled the Orang and the Chimpanzee in stature, and appears to have come near the Gibbons, while the latter belonged to the group of long-tailed Indian monkeys.¹

No one will suppose that the species described as occurring in the Miocene rocks of Switzerland, represent more than a fragment of a much larger fauna that inhabited that and other regions of the old continent, of which our own area then formed part, and it is impossible to believe that with a teeming fauna in the lands to the east and south, a portion of it, now changed into the British Islands,

¹ This epitome of the Miocene fauna is condensed from 'The Primeval World of Switzerland,' by Professor Heer, of the University of Zurich, edited by James Heywood, M.A., F.R.S., a most interesting book and worthily translated.

[268 Physical Geography.]

should have been destitute of all mammalian life, for it seems more than absurd to suppose, that none of these animals should have found their way into our area, while they swarmed in regions so near as Switzerland, France, and the Rhine, where, however, at that time no Rhine existed. On the contrary, I, for one, take it for granted that some of them must have inhabited the southern ground of our British area, where the old lake of Bovey Tracey lay in a latitude not two degrees further north than that old lake of the Valley of the Rhine, which in those days, between the mountains of the Black Forest and the Vosges, stretched all the way from Basle to Mayence and the neighbourhood of Bingen. The banks of that lake were inhabited by the same mammalia that inhabits the adjoining area of the great Miocene Swiss lakes, and we may readily believe that, in the physiography of the south British area, there was nothing inimical to the thriving of such species, for its climate was then warm, and its great plains, tablelands, valleys, and mountains, were doubtless clothed with a rich vegetation. This, however, we may assume, that, just as we pass northward, the vegetation of the day assumed a more northern type, so in the mountain land of that older Scotland, and on its western flanks, where lofty volcanoes were growing, the fauna would get mingled with northern forms, all of which seem to be lost to us even in a fossil state, the physical conditions of the British area having been of a kind, that no broad and thick sediments were deposited in which the bones of mammals could be preserved.

It is, however, possible, and indeed probable, that we get a glimpse of part of this mammalian life preserved in a curiously mingled fauna, the remains of which lie buried at the base of various members of the Crag. I

[Physical Geography. 269]

have, therefore, gone into the subject of the Miocene fauna of the Continent in a way that at first sight may seem out of place in this book, but which in reality is not so, for it throws a reflected light on the assemblage of animals which there is good reason to believe partly inhabited our area during the same epoch.