The Student's Elements of Geology Part 25

Not a few of the other mammalia of the Limagne belong undoubtedly to genera and species elsewhere proper to the Lower Miocene. Thus, for example, the Cainotherium of Bravard, a genus not far removed from the Anoplotherium, is represented by several species, one of which, as I learn from Mr. Waterhouse, agrees with Microtherium Renggeri of the Mayence basin. In like manner, the Amphitragulus elegans of Pomel, an Auvergne fossil, is identified by Waterhouse with Dorcatherium nanum of Kaup, a Rhenish species from Weissenau, near Mayence.

A small species, also, of rodent, of the genus t.i.tanomys of H. von Meyer, is common to the Lower Miocene of Mayence and the Limagne d'Auvergne, and there are many other points of agreement which the discordance of nomenclature tends to conceal. A remarkable carnivorous genus, the Hyaenodon of Laizer, is represented by more than one species. The same genus has also been found in the Upper Eocene marls of Hordwell Cliff, Hampshire, just below the level of the Bembridge Limestone, and therefore a formation older than the Gypsum of Paris. Several species of opossum (Didelphis) are met with in the same strata of the Limagne.

The total number of mammalia enumerated by M. Pomel as appertaining to the Lower Miocene fauna of the Limagne and Velay falls little short of a hundred, and with them are a.s.sociated some large crocodiles and tortoises, and some Ophidian and Batrachian reptiles.

LOWER MOLa.s.sE OF SWITZERLAND.

The two upper divisions of the Swiss Mola.s.se-- the one fresh-water, the other marine-- have already been described in the preceding chapter. I shall now proceed to treat of the third division, which is of Lower Miocene age. Nearly the whole of this Lower Mola.s.se is fresh-water, yet some of the inferior beds contain a mixture of marine and fluviatile sh.e.l.ls, the Cerithium margaritaceum, a well-known Lower Miocene fossil, being one of the marine species.

Notwithstanding, therefore, that some of these Lower Miocene strata consist of old shingle-beds several thousand feet in thickness, as in the Rigi, near Lucerne, and in the Speer, near Wesen, mountains 5000 and 7000 feet above the sea, the deposition of the whole series must have begun at or below the sea- level.

The conglomerates, as might be expected, are often very unequal in thickness, in closely adjoining districts, since in a littoral formation acc.u.mulations of pebbles would swell out in certain places where rivers entered the sea, and would thin out to comparatively small dimensions where no streams or only small ones came down to the coast. For ages, in spite of a gradual depression of the land and adjacent sea-bottom, the rivers continued to cover the sinking area with their deltas; until finally, the subsidence being in excess, the sea of the Middle Mola.s.se gained upon the land, and marine beds were thrown down over the dense ma.s.s of fresh-water and brackish-water deposit, called the Lower Mola.s.se, which had previously acc.u.mulated.

FLORA OF THE LOWER MOLa.s.sE.

In part of the Swiss Mola.s.se, which belongs exclusively to the Lower Miocene period, the number of plants has been estimated at more than 500 species, somewhat exceeding those which were before enumerated as occurring in the two upper divisions. The Swiss Lower Miocene may best be studied on the northern borders of the Lake of Geneva, between Lausanne and Vevay, where the contiguous villages of Monod and Rivaz are situated. The strata there, which I have myself examined, consist of alternations of conglomerate, sandstone, and finely laminated marls with fossil plants. A small stream falls in a succession of cascades over the harder beds of pudding-stone, which resist, while the sandstone and plant-bearing shales and marls give way. From the latter no less than 193 species of plants have been obtained by the exertions of MM. Heer and Gaudin, and they are considered to afford a true type of the vegetation of the Lower Miocene formations of Switzerland-- a vegetation departing farther in its character from that now flourishing in Europe than any of the higher members of the series before alluded to, and yet displaying so much affinity to the flora of Oeningen as to make it natural for the botanist to refer the whole to one and the same Miocene period. There are, indeed, no less than 81 species of these Older Miocene plants which pa.s.s up into the flora of Oeningen.

This fact is important as bearing on the propriety of cla.s.sing the Lower Mola.s.se of Switzerland as belonging to the Miocene rather than to the latter part of the Eocene period. There are, indeed, so many types among the fossils, both specific and generic, which have a wide range through the whole of the Mola.s.se, that a unity of character is thereby stamped on the whole flora, in spite of the contrast between the plants of the uppermost and lowest formations, or between Oeningen and Monod. The proofs of a warmer climate, and the excess of arborescent over herbaceous plants, and of evergreen trees over deciduous species, are characters common to the whole flora, but which are intensified as we descend to the inferior deposits.

(FIGURE 151. Sabal major, Unger sp. Vevay. Lower Miocene; Heer, Plate 41.)

Nearly all the plants at Monod are contained in three layers of marl separated by two of soft sandstone. The thickness of the marls is ten feet, and vegetable matter predominates so much in some layers as to form an imperfect lignite. One bed is filled with large leaves of a species of fig (Ficus populina), and of a hornbeam (Carpinus grandis), the strength of the wind having probably been great when they were blown into the lake; whereas another contiguous layer contains almost exclusively smaller leaves, indicating, apparently, a diminished strength in the wind. Some of the upper beds at Monod abound in leaves of Proteaceae, Cyperaceae, and ferns, while in some of the lower ones Sequoia, Cinnamomum, and Sparganium are common. In one bed of sandstone the trunk of a large palm-tree was found unaccompanied by other fossils, and near Vevay, in the same series of Lower Miocene strata, the leaves of a palm of the genus Sabal (Figure 151), a genus now proper to America, were obtained.

Among other genera of the same cla.s.s is a Flabellaria occurring near Lausanne, and a magnificent Phoenicites allied to the date palm. When these plants flourished the climate must have been much hotter than now. The Alps were no doubt much lower, and the palms now found fossil in strata elevated 2000 feet above the sea grew nearly at the sea-level, as is demonstrated by the brackish- water character of some of the beds into which they were carried by winds or rivers from the adjoining coast.

(FIGURE 152. Banksia.

a. Fruit of fossil Banksia.

b. Leaf of Banksia Deekiana.)

In the same plant-bearing deposits of the Lower Mola.s.se in Switzerland leaves have been found which have been ascribed to the order Proteaceae already spoken of as well represented in the Oeningen beds (see Chapter 14). The Proteas and other plants of this family now flourish at the Cape of Good Hope; while the Banksias, and a set of genera distinct from those of Africa, grow most luxuriantly in the southern and temperate parts of Australia. They were probably inhabitants, says Heer, of dry hilly ground, and the stiff leathery character of their leaves must have been favourable to their preservation, allowing them to float on a river for great distances without being injured, and then to sink, when water-logged, to the bottom. It has been objected that the fruit of the Proteaceae is of so tough and enduring a texture that it ought to have been more commonly met with; but in the first place we must not forget the numerous cones found in the Eocene strata of Sheppey, which all admit to be proteaceous and to belong to at least two species (see Chapter 14). Secondly, besides the fruit of Hakea before mentioned (Chapter 14), Heer found a.s.sociated with fossil leaves, having the exact form and nervation of Banksia, fruit precisely such as may have come from a cone of that plant, and lately he has received another similar fruit from the Lower Miocene strata of Lucerne. They may have fallen out of a decayed cone in the same way as often happens to the seeds of the spruce fir, Pinus abies, found scattered over the ground in our woods. It is a known fact that among the living Proteaceae the cones are very firmly attached to the branches, so that the seeds drop out without the cone itself falling to the ground, and this may perhaps be the reason why, in some instances in which fossil seeds have been found, no traces of the cone have been observed.

(FIGURE 153. Sequoia Langsdorfii. Ad. Brong., 1/3 natural size. Rivaz, near Lausanne; Heer, Plate 21 Figure 4. Upper and Lower Miocene and Lower Pliocene, Val d'Arno.

a. Branch with leaves.

b. Young cone.)

Among the Coniferae the Sequoia here figured is common at Rivaz, and is one of the most universal plants in the Lowest Miocene of Switzerland, while it also characterises the Miocene Brown Coals of Germany and certain beds of the Val d'Arno, which I have called Older Pliocene, Chapter 13.

(FIGURE 154. Lastraea stiriaca, Unger; Heer's Flora, Plate 143 Figure 8. Natural size. Lower and Upper Miocene, Switzerland.

a. Specimen from Monod, showing the position of the sori on the middle of the tertiary nerves.

b. More common appearance, where the sori remain and the nerves are obliterated.)

Among the ferns met with in profusion at Monod is the Lastraea stiriaca, Unger, which has a wide range in the Miocene period from strata of the age of Oeningen to the lowest part of the Swiss Mola.s.se. In some specimens, as shown in Figure 154, the fructification is distinctly seen.

(FIGURE 155. Cinnamomum Rossma.s.sleri, Heer. Daphnogene cinnamomifolia, Unger.

Upper and Lower Miocene, Switzerland and Germany.)

Among the laurels several species of Cinnamomum are very conspicuous. Besides the C. polymorphum, before figured, Chapter 14, another species also ranges from the Lower to the Upper Mola.s.se of Switzerland, and is very characteristic of different deposits of Brown Coal in Germany. It has been called Cinnamomum Rossma.s.sleri by Heer (see Figure 155). The leaves are easily recognised as having two side veins, which run up uninterruptedly to their point.

AMERICAN CHARACTER OF THE FLORA.

If we consider not merely the number of species but those plants which const.i.tute the ma.s.s of the Lower Miocene vegetation, we find the European part of the fossil flora very much less prominent than in the Oeningen beds, while the foreground is occupied by American forms, by evergreen oaks, maples, poplars, planes, Liquidambar, Robinia, Sequoia, Taxodium, and ternate-leaved pines. There is also a much greater fusion of the characters now belonging to distinct botanical provinces than in the Upper Miocene flora, and we shall find this fusion still more strikingly exemplified as we go back to the antecedent Eocene and Cretaceous periods.

Professor Heer has advocated the doctrine, first advanced by Unger to explain the large number of American genera in the Miocene flora of Europe, that the present basin of the Atlantic was occupied by land over which the Miocene flora could pa.s.s freely. But other able botanists have shown that it is far more probable that the American plants came from the east and not from the west, and instead of reaching Europe by the shortest route over an imaginary Atlantis, migrated in an opposite direction, crossing the whole of Asia.

ARCTIC MIOCENE FLORA.

But when we indulge in speculations as to the geographical origin of the Miocene plants of Central Europe, we must take into account the discoveries recently made of a rich terrestrial flora having flourished in the Arctic Regions in the Miocene period from which many species may have migrated from a common centre so as to reach the present continents of Europe, Asia, and America. Professor Heer has examined the various collections of fossil plants that have been obtained in North Greenland (lat.i.tude 70 degrees), Iceland, Spitzbergen, and other parts of the Arctic regions, and has determined that they are of Miocene age and indicate a temperate climate. (Heer "Miocene baltische Flora" and "Fossil-flora von Alaska" 1869.) Including the collections recently brought from Greenland by Mr.

Whymper, the Arctic Miocene flora now comprises 194 species, and that of Greenland 137 species, of which 46, or exactly one-third, are identical with plants found in the Miocene beds of Central Europe. Considerably more than half the number are trees, which is the more remarkable since, at the present day, trees do not exist in any part of Greenland even 10 degrees farther south.

More than thirty species of Coniferae have been found, including several Sequoias (allied to the gigantic Wellingtonia of California), with species of Thujopsis and Salisburia now peculiar to j.a.pan. There are also beeches, oaks, planes, poplars, maples, walnuts, limes, and even a magnolia, two cones of which have recently been obtained, proving that this splendid evergreen not only lived but ripened its fruit within the Arctic circle. Many of the limes, planes, and oaks were large-leaved species, and both flowers and fruit, besides immense quant.i.ties of leaves, are in many cases preserved. Among the shrubs were many evergreens, as Andromeda, and two extinct genera, Daphnogene and M'Clintockia, with fine leathery leaves, together with hazel, blackthorn, holly, logwood, and hawthorn. A species of Zamia (Zamites) grew in the swamps, with Potamogeton, Sparganium, and Menyanthes, while ivy and vines twined around the forest trees and broad-leaved ferns grew beneath their shade. Even in Spitzbergen, as far north as lat.i.tude 78 degrees 56', no less than ninety-five species of fossil plants have been obtained, including Taxodium of two species, hazel, poplar, alder, beech, plane-tree, and lime. Such a vigorous growth of trees within 12 degrees of the pole, where now a dwarf willow and a few herbaceous plants form the only vegetation, and where the ground is covered with almost perpetual snow and ice, is truly remarkable.

The ident.i.ty of so many of the fossils with Miocene species of Central Europe and Italy not only proves that the climate of Greenland was much warmer than it is now, but also renders it probable that a much more uniform climate prevailed over the entire northern hemisphere. This is also indicated by the whole character of the Upper Miocene flora of Central Europe, which does not necessitate a mean temperature very much greater than exists at present, if we suppose such absence of winter cold as is proper to insular climates. Professor Heer believes that the mean temperature of North Greenland must have been at least 30 degrees higher than at present, while an addition of 10 degrees to the mean temperature of Central Europe would probably be as much as was required.

The chief locality where this wonderful flora is preserved is at Atanekerdluk in North Greenland (lat.i.tude 70 degrees), on a hill at an elevation of about 1200 feet above the sea. There is here a considerable succession of sedimentary strata pierced by volcanic rocks. Fossil plants occur in all the beds, and the erect trunks as thick as a man's body which are sometimes found, together with the abundance of specimens of flowers and fruit in good preservation, sufficiently prove that the plants grew where they are now found. At Disco island and other localities on the same part of the coast, good coal is abundant, interstratified with beds of sandstone, in some of which fossil plants have also been found, similar to those at Atanekerdluk.

LOWER MIOCENE, BELGIUM.

(FIGURE 156. Leda (Nucula) Deshayesiana, Nyst.)

(FIGURE 157. Vanessa pluto; natural size. Lower Miocene, Radaboj, Croatia.)

The Upper Miocene Bolderberg beds, mentioned in Chapter 14, rest on a Lower Miocene formation called the Rupelian of Dumont. This formation is best seen at the villages of Rupelmonde and Boom, ten miles south of Antwerp, on the banks of the Scheldt and near the junction with it of a small stream called the Rupel. A stiff clay abounding in fossils is extensively worked at the above localities for making tiles. It attains a thickness of about 100 feet, and though very different in age, much resembles in mineral character the "London clay,"

containing, like it, septaria or concretions of argillaceous limestone traversed by cracks in the interior, which are filled with calc-spar. The sh.e.l.ls, referable to about forty species, have been described by MM. Nyst and De Koninck. Among them Leda (or Nucula) Deshayesiana (see Figure 156) is by far the most abundant; a fossil unknown as yet in the English tertiary strata, but when young much resembling Leda amygdaloides of the London Clay proper (see Figure 213 Chapter 16). Among other characteristic sh.e.l.ls are Pecten Hoeninghausii, and a species of Ca.s.sidaria, and several of the genus Pleurotoma. Not a few of these testacea agree with English Eocene species, such as Actaeon simulatus, Sowb, Cancellaria evulsa, Brander, Corbula pisum (Figure 157), and Nautilus (Aturia) ziczac. They are accompanied by many teeth of sharks, as Lamna contortidens, Ag., Oxyrhinaxiphodon, Ag., Carcharodon angustidens (see Figure 196 Chapter 16), Ag., and other fish, some of them common to the Middle Eocene strata.

KLEYN SPAWEN BEDS.

The succession of the Lower Miocene strata of Belgium can be best studied in the environs of Kleyn Spawen, a village situated about seven miles west of Maestricht, in the old province of Limburg in Belgium. In that region, about 200 species of testacea, marine and fresh-water, have been obtained, with many foraminifera and remains of fish. In none of the Belgian Lower Miocene strata could I find any nummulites; and M. d'Archiac had previously observed that these foraminifera characterise his "Lower Tertiary Series," as contrasted with the Middle, and they therefore serve as a good test of age between Eocene and Miocene, at least in Belgium and the North of France. (D'Archiac Monograph pages 79, 100.) Between the Bolderberg beds and the Rupelian clay there is a great gap in Belgium, which seems, according to M. Beyrich, to be filled up in the North of Germany by what he calls the Sternberg beds, and which, had Dumont found them in Belgium, he might probably have termed Upper Rupelian.

LOWER MIOCENE OF GERMANY.

RUPELIAN CLAY OF HERMSDORF, NEAR BERLIN.

Professor Beyrich has described a ma.s.s of clay, used for making tiles, within seven miles of the gates of Berlin, near the village of Hermsdorf, rising up from beneath the sands with which that country is chiefly overspread. This clay is more than forty feet thick, of a dark bluish-grey colour, and, like that of Rupelmonde, contains septaria. Among other sh.e.l.ls, the Leda Deshayesiana, before mentioned (Figure 156), abounds, together with many species of Pleurotoma, Voluta, etc., a certain proportion of the fossils being identical in species with those of Rupelmonde.

MAYENCE BASIN.

An elaborate description has been published by Dr. F. Sandberger of the Mayence tertiary area, which occupies a tract from five to twelve miles in breadth, extending for a great distance along the left bank of the Rhine from Mayence to the neighbourhood of Manheim, and which is also found to the east, north, and south-west of Frankfort. M. De Koninck, of Liege, first pointed out to me that the purely marine portion of the deposit contained many species of sh.e.l.ls common to the Kleyn Spawen beds, and to the clay of Rupelmonde, near Antwerp. Among these he mentioned Ca.s.sidaria depressa, Tritonium argutum, Brander (T.

flandric.u.m, De Koninck), Tornatella simulata, Aporrhais Sowbyi, Leda Deshayesiana (Figure 156), Corbula pisum, (Figure 158) and others.

LOWER MIOCENE BEDS OF CROATIA.

The Brown Coal of Radaboj, near Angram in Croatia, not far from the borders of Styria, is covered, says Von Buch, by beds containing the marine sh.e.l.ls of the Vienna basin, or, in other words, by Upper Miocene or Falunian strata. They appear to correspond in age to the Mayence basin, or to the Rupelian strata of Belgium. They have yielded more than 200 species of fossil plants, described by the late Professor Unger. These plants are well preserved in a hard marlstone, and contain several palms; among them the Sabal, Figure 151, and another genus allied to the date-palm Phoenicites spectabilis. The only abundant plant among the Radaboj fossils which is characteristic of the Upper Miocene period is the Populus mutabilis, whereas no less than fifty of the Radaboj species are common to the more ancient flora of the Lower Mola.s.se of Switzerland.

The insect fauna is very rich, and, like the plants, indicates a more tropical climate than do the fossils of Oeningen presently to be mentioned. There are ten species of Termites, or white ants, some of gigantic size, and large dragon- flies with speckled wings, like those of the Southern States in North America; there are also gra.s.shoppers of considerable size, and even the Lepidoptera are not unrepresented. In one instance, the pattern of a b.u.t.terfly's wing has escaped obliteration in the marl-stone of Radaboj; and when we reflect on the remoteness of the time from which it has been faithfully transmitted to us, this fact may inspire the reader with some confidence as to the reliable nature of the characters which other insects of a more durable texture, such as the beetles, may afford for specific determination. The Vanessa above figured retains, says Heer, some of its colours, and corresponds with Vanessa Hadena of India.

Professor Beyrich has made known to us the existence of a long succession of marine strata in North Germany, which lead by an almost gradual transition from beds of Upper Miocene age to others of the age of the base of the Lower Miocene.

Although some of the German lignites called Brown Coal belong to the upper parts of this series, the most important of them are of Lower Miocene date, as, for example, those of the Siebengebirge, near Bonn, which are a.s.sociated with volcanic rocks.

Professor Beyrich confines the term "Miocene" to those strata which agree in age with the faluns of Touraine, and he has proposed the term "Oligocene" for those older formations called Lower Miocene in this work.

LOWER MIOCENE OF ITALY.

In the hills of which the Superga forms a part there is a great series of Tertiary strata which pa.s.s downward into the Lower Miocene. Even in the Superga itself there are some fossil plants which, according to Heer, have never been found in Switzerland so high as the marine Mola.s.se, such as Banksia longifolia, and Carpinus grandis. In several parts of the Ligurian Apennines, as at Dego and Carcare, the Lower Miocene appears, containing some nummulites, and at Cadibona, north of Savona, fresh-water strata of the same age occur, with dense beds of lignite inclosing remains of the Anthracotherium magnum and Anthracotherium minimum, besides other mammalia enumerated by Gastaldi. In these beds a great number of the Lower Miocene plants of Switzerland have been discovered.