The Student's Elements of Geology Part 22

There is probably no part of Europe where the Newer Pliocene formations enter so largely into the structure of the earth's crust, or rise to such heights above the level of the sea, as Sicily. They cover nearly half the island, and near its centre, at Castrogiovanni, reach an elevation of 3000 feet. They consist princ.i.p.ally of two divisions, the upper calcareous and the lower argillaceous, both of which may be seen at Syracuse, Girgenti, and Castrogiovanni. According to Philippi, to whom we are indebted for the best account of the tertiary sh.e.l.ls of this island, thirty-five species out of one hundred and twenty-four obtained from the beds in central Sicily are extinct.

A geologist, accustomed to see nearly all the Newer Pliocene formations in the north of Europe occupying low grounds and very incoherent in texture, is naturally surprised to behold formations of the same age so solid and stony, of such thickness, and attaining so great an elevation above the level of the sea.

The upper or calcareous member of this group in Sicily consists in some places of a yellowish-white stone, like the Calcaire Grossier of Paris; in others, of a rock nearly as compact as marble. Its aggregate thickness amounts sometimes to 700 or 800 feet. It usually occurs in regular horizontal beds, and is occasionally intersected by deep valleys, such as those of Sortino and Pentalica, in which are numerous caverns. The fossils are in every stage of preservation, from sh.e.l.ls retaining portions of their animal matter and colour to others which are mere casts. The limestone pa.s.ses downward into a sandstone and conglomerate, below which is clay and blue marl, from which perfect sh.e.l.ls and corals may be disengaged. The clay sometimes alternates with yellow sand.

South of the plain of Catania is a region in which the tertiary beds are intermixed with volcanic matter, which has been for the most part the product of submarine eruptions. It appears that, while the clay, sand, and yellow limestone before mentioned were in course of deposition at the bottom of the sea, volcanoes burst out beneath the waters, like that of Graham Island, in 1831, and these explosions recurred again and again at distant intervals of time. Volcanic ashes and sand were showered down and spread by the waves and currents so as to form strata of tuff, which are found intercalated between beds of limestone and clay containing marine sh.e.l.ls, the thickness of the whole ma.s.s exceeding 2000 feet. The fissures through which the lava rose may be seen in many places, forming what are called DIKES.

(FIGURE 133. Pecten jacobaeus; half natural size.)

No sh.e.l.l is more conspicuous in these Sicilian strata than the great scallop, Pecten jacobaeus (Figure 133), now so common in the neighbouring seas. The more we reflect on the preponderating number of this and other recent sh.e.l.ls, the more we are surprised at the great thickness, solidity, and height above the sea of the rocky ma.s.ses in which they are entombed, and the vast amount of geographical change which has taken place since their origin. It must be remembered that, before they began to emerge, the uppermost strata of the whole must have been deposited under water. In order, therefore, to form a just conception of their antiquity, we must first examine singly the innumerable minute parts of which the whole is made up, the successive beds of sh.e.l.ls, corals, volcanic ashes, conglomerates, and sheets of lava; and we must afterwards contemplate the time required for the gradual upheaval of the rocks, and the excavation of the valleys. The historical period seems scarcely to form an appreciable unit in this computation, for we find ancient Greek temples, like those of Girgenti (Agrigentum), built of the modern limestone of which we are speaking, and resting on a hill composed of the same; the site having remained to all appearances unaltered since the Greeks first colonised the island.

It follows, from the modern geological date of these rocks, that the fauna and flora of a large part of Sicily are of higher antiquity than the country itself.

The greater part of the island has been raised above the sea since the epoch of existing species, and the animals and plants now inhabiting it must have migrated from adjacent countries, with whose productions the species are now identical. The average duration of species would seem to be so great that they are destined to outlive many important changes in the configuration of the earth's surface, and hence the necessity for those innumerable contrivances by which they are enabled to extend their range to new lands as they are formed, and to escape from those which sink beneath the sea.

NEWER PLIOCENE STRATA OF THE UPPER VAL D'ARNO.

When we ascend the Arno for about ten miles above Florence, we arrive at a deep narrow valley called the Upper Val d'Arno, which appears once to have been a lake, at a time when the valley below Florence was an arm of the sea. The horizontal lacustrine strata of this upper basin are twelve miles long and two broad. The depression which they fill has been excavated out of Eocene and Cretaceous rocks, which form everywhere the sides of the valley in highly inclined stratification. The thickness of the more modern and unconformable beds is about 750 feet, of which the upper 200 feet consist of Newer Pliocene strata, while the lower are Older Pliocene. The newer series are made up of sands and a conglomerate called "sansino." Among the imbedded fossil mammalia are Mastodon arvernensis, Elephas meridionalis, Rhinoceros etruscus, Hippopotamus major, and remains of the genera bear, hyaena, and felis, nearly all of which occur in the Cromer forest-bed (see Chapter 13).

In the same upper strata are found, according to M. Gaudin, the leaves and cones of Glyptostrobus europaeus, a plant closely allied to G. heterophyllus, now inhabiting the north of China and j.a.pan. This conifer had a wide range in time, having been traced back to the Lower Miocene strata of Switzerland, and being common at Oeningen in the Upper Miocene, as we shall see in the sequel (Chapter 14.)

OLDER PLIOCENE OF ITALY.-- SUBAPENNINE STRATA.

The Apennines, it is well-known, are composed chiefly of Secondary or Mesozoic rocks, forming a chain which branches off from the Ligurian Alps and pa.s.ses down the middle of the Italian peninsula. At the foot of these mountains, on the side both of the Adriatic and the Mediterranean, are found a series of tertiary strata, which form, for the most part, a line of low hills occupying the s.p.a.ce between the older chain and the sea. Brocchi was the first Italian geologist who described this newer group in detail, giving it the name of the Subapennine.

Though chiefly composed of Older Pliocene strata, it belongs, nevertheless, in part, both to older and newer members of the tertiary series. The strata, for example, of the Superga, near Turin, are Miocene; those of Asti and Parma Older Pliocene, as is the blue marl of Sienna; while the sh.e.l.ls of the inc.u.mbent yellow sand of the same territory approach more nearly to the recent fauna of the Mediterranean, and may be Newer Pliocene.

We have seen that most of the fossil sh.e.l.ls of the Older Pliocene strata of Suffolk which are of recent species are identical with testacea now living in British seas, yet some of them belong to Mediterranean species, and a few even of the genera are those of warmer climates. We might therefore expect, in studying the fossils of corresponding age in countries bordering the Mediterranean, to find among them some species and genera of warmer lat.i.tudes.

Accordingly, in the marls belonging to this period at Asti, Parma, Sienna, and parts of the Tuscan and Roman territories, we observe the genera Conus, Cypraea, Strombus, Pyrula, Mitra, Fasciolaria, Sigaretus, Delphinula, Ancillaria, Oliva, Terebellum, Terebra, Perna, Plicatula, and Corbis, some characteristic of tropical seas, others represented by species more numerous or of larger size than those now proper to the Mediterranean.

OLDER PLIOCENE FLORA OF ITALY.

(FIGURE 134. Oreodaphne Heerii.

Leaf half natural size. (Feuilles fossiles de la Toscane.))

I have already alluded to the Newer Pliocene deposits of the Upper Val d'Arno above Florence, and stated that below those sands and conglomerates, containing the remains of the Elephas meridionalis and other a.s.sociated quadrupeds, lie an older horizontal and conformable series of beds, which may be cla.s.sed as Older Pliocene. They consist of blue clays with some subordinate layers of lignite, and exhibit a richer flora than the overlying Newer Pliocene beds, and one receding farther from the existing vegetation of Europe. They also comprise more species common to the antecedent Miocene period. Among the genera of flowering plants, M. Gaudin enumerates pine, oak, evergreen oak, plum, plane, alder, elm, fig, laurel, maple, walnut, birch, buckthorn, hickory, sumach, sarsaparilla, sa.s.safras, cinnamon, Glyptostrobus, Taxodium, Sequoia, Persea, Oreodaphne (Figure 134), Ca.s.sia, and Psoralea, and some others. This a.s.semblage of plants indicates a warm climate, but not so subtropical an one as that of the Upper Miocene period, which will presently be considered.

(FIGURE 135. Liquidambar europaeum, var. trilobatum, A. Br. (sometimes four- lobed, and more commonly five-lobed).

a. Leaf, half natural size.

b. Part of same, natural size.

c. Fruit, natural size.

d. Seed, natural size. Oeningen.)

M. Gaudin, jointly with the Marquis Strozzi, has thrown much light on the botany of beds of the same age in another part of Tuscany, at a place called Montajone, between the rivers Elsa and Evola, where, among other plants, is found the Oreodaphne Heerii, Gaud. (See Figure 134), which is probably only a variety of Oreodaphne foetens, or the laurel called the Til in Madeira, where, as in the Canaries, it const.i.tutes a large portion of the native woods, but can not now endure the climate of Europe. In the fossil specimens the same glands or protuberances are preserved (see Figure 134) as those which are seen in the axils of the primary veins of the leaves in the recent Til. (Contributions a la Flore fossile Italienne. Gaudin and Strozzi. Plate 11 Figure 3. Gaudin page 22.) Another plant also indicating a warmer climate is the Liquidambar europaeum, Brong. (see Figure 135), a species nearly allied to L. styracifluum, L., which flourishes in most places in the Southern States of North America, on the borders of the Gulf of Mexico.

CHAPTER XIV.

MIOCENE PERIOD.-- UPPER MIOCENE.

Upper Miocene Strata of France.-- faluns of Touraine.

Tropical Climate implied by Testacea.

Proportion of recent Species of Sh.e.l.ls.

faluns more ancient than the Suffolk Crag.

Upper Miocene of Bordeaux and the South of France.

Upper Miocene of Oeningen, in Switzerland.

Plants of the Upper Fresh-water Mola.s.se.

Fossil Fruit and Flowers as well as Leaves.

Insects of the Upper Mola.s.se.

Middle or Marine Mola.s.se of Switzerland.

Upper Miocene Beds of the Bolderberg, in Belgium.

Vienna Basin.

Upper Miocene of Italy and Greece.

Upper Miocene of India; Siwalik Hills.

Older Pliocene and Miocene of the United States.

UPPER MIOCENE STRATA OF FRANCE.-- FALUNS OF TOURAINE.

The strata which we meet with next in the descending order are those called by many geologists "Middle Tertiary," for which in 1833 I proposed the name of Miocene, selecting the "faluns" of the valley of the Loire, in France, as my example or type. I shall now call these falunian deposits Upper Miocene, to distinguish them from others to which the name of Lower Miocene will be given.

No British strata have a distinct claim to be regarded as Upper Miocene, and as the Lower Miocene are also but feebly represented in the British Isles, we must refer to foreign examples in ill.u.s.tration of this important period in the earth's history. The term "faluns" is given provincially by French agriculturists to sh.e.l.ly sand and marl spread over the land in Touraine, just as similar sh.e.l.ly deposits were formerly much used in Suffolk to fertilise the soil, before the coprolitic or phosphatic nodules came into use. Isolated ma.s.ses of such faluns occur from near the mouth of the Loire, in the neighbourhood of Nantes, to as far inland as a district south of Tours. They are also found at Pontlevoy, on the Cher, about seventy miles above the junction of that river with the Loire, and thirty miles south-east of Tours. Deposits of the same age also appear under new mineral conditions near the towns of Dinan and Rennes, in Brittany. I have visited all the localities above enumerated, and found the beds on the Loire to consist princ.i.p.ally of sand and marl, in which are sh.e.l.ls and corals, some entire, some rolled, and others in minute fragments. In certain districts, as at Doue, in the Department of Maine and Loire, ten miles south- west of Saumur, they form a soft building-stone, chiefly composed of an aggregate of broken sh.e.l.ls, bryozoa, corals, and echinoderms, united by a calcareous cement; the whole ma.s.s being very like the Coralline Crag near Aldborough, and Sudbourn in Suffolk. The scattered patches of faluns are of slight thickness, rarely exceeding fifty feet; and between the district called Sologne and the sea they repose on a great variety of older rocks; being seen to rest successively upon gneiss, clay-slate, various secondary formations, including the chalk; and, lastly, upon the upper fresh-water limestone of the Parisian tertiary series, which, as before mentioned (Chapter 9), stretches continuously from the basin of the Seine to that of the Loire.

(FIGURE 136. Dinotherium giganteum, Kaup.)

At some points, as at Louans, south of Tours, the sh.e.l.ls are stained of a ferruginous colour, not unlike that of the Red Crag of Suffolk. The species are, for the most part, marine, but a few of them belong to land and fluviatile genera. Among the former, Helix turonensis (Figure 38, Chapter 3) is the most abundant. Remains of terrestrial quadrupeds are here and there intermixed, belonging to the genera Dinotherium (Figure 136), Mastodon, Rhinoceros, Hippopotamus, Chaeropotamus, Dichobune, Deer, and others, and these are accompanied by cetacea, such as the Lamantin, Morse, Sea-calf, and Dolphin, all of extinct species.

The fossil testacea of the faluns of the Loire imply, according to the late Edward Forbes, that the beds were formed partly on the sh.o.r.e itself at the level of low water, and partly at very moderate depths, not exceeding ten fathoms below that level. The molluscan fauna is, on the whole, much more littoral than that of the Pliocene Red and Coralline Crag of Suffolk, and implies a shallower sea. It is, moreover, contrasted with the Suffolk Crag by the indications it affords of an extra-European climate. Thus it contains seven species of Cypraea, some larger than any existing cowry of the Mediterranean, several species of Oliva, Ancillaria, Mitra, Terebra, Pyrula, Fasciolaria, and Conus. Of the cones there are no less than eight species, some very large, whereas the only European cone now living is of diminutive size. The genus Nerita, and many others, are also represented by individuals of a type now characteristic of equatorial seas, and wholly unlike any Mediterranean forms. These proofs of a more elevated temperature seem to imply the higher antiquity of the faluns as compared with the Suffolk Crag, and are in perfect accordance with the fact of the smaller proportion of testacea of recent species found in the faluns.

Out of 290 species of sh.e.l.ls, collected by myself in 1840 at Pontlevoy, Louans, Bossee, and other villages twenty miles south of Tours, and at Savigne, about fifteen miles north-west of that place, seventy-two only could be identified with recent species, which is in the proportion of twenty-five per cent. A large number of the 290 species are common to all the localities, those peculiar to each not being more numerous than we might expect to find in different bays of the same sea.

The total number of species of testaceous mollusca from the faluns in my possession is 302, of which forty-five only, or fourteen per cent, were found by Mr. Wood to be common to the Suffolk Crag. The number of corals, including bryozoa and zoantharia, obtained by me at Doue and other localities before adverted to, amounts to forty-three, as determined by Mr. Lonsdale, of which seven (one of them a zoantharian) agree specifically with those of the Suffolk Crag. Some of the genera occurring fossil in Touraine, as the corals Astrea and Dendrophyllia, and the bryozoan Lunulites, have not been found in European seas north of the Mediterranean; nevertheless, the zoantharia of the faluns do not seem to indicate, on the whole, so warm a climate as would be inferred from the sh.e.l.ls.

It was stated that, on comparing about 300 species of Touraine sh.e.l.ls with about 450 from the Suffolk Crag, forty-five only were found to be common to both, which is in the proportion of only fifteen per cent. The same small amount of agreement is found in the corals also. I formerly endeavoured to reconcile this marked difference in species with the supposed co-existence of the two faunas, by imagining them to have severally belonged to distinct zoological provinces or two seas, the one opening to the north and the other to the south, with a barrier of land between them, like the Isthmus of Suez, now separating the Red Sea and the Mediterranean. But I now abandon that idea for several reasons; among others, because I succeeded in 1841 in tracing the Crag fauna southward in Normandy to within seventy miles of the Falunian type, near Dinan, yet found that both a.s.semblages of fossils retained their distinctive characters, showing no signs of any blending of species or transition of climate.

The princ.i.p.al grounds, however, for referring the English Crag to the older Pliocene and the French faluns to the Upper Miocene epochs, consist in the predominance of fossil sh.e.l.ls in the British strata identifiable with species not only still living, but which are now inhabitants of neighbouring seas, while the accompanying extinct species are of genera such as characterise Europe. In the faluns, on the contrary, the recent species are in a decided minority; and most of them are now inhabitants of the Mediterranean, the coast of Africa, and the Indian Ocean; in a word, less northern in character, and pointing to the prevalence of a warmer climate. They indicate a state of things receding farther from the present condition of Central Europe in physical geography and climate, and doubtless, therefore, receding farther from our era in time.

(FIGURE 137. Voluta Lamberti, Sowerby. Variety characteristic of Faluns of Touraine. Miocene.)

Among the conspicuous fossils common to the faluns of the Loire and the Suffolk Crag is a variety of the Voluta Lamberti, a sh.e.l.l already alluded to (Figure 123). The specimens of this sh.e.l.l which I have myself collected in Touraine, or have seen in museums, are thicker and heavier than British individuals of the same species, and shorter in proportion to their width, and have the folds on the columella less oblique, as represented in Figure 137.

UPPER MIOCENE OF BORDEAUX AND THE SOUTH OF FRANCE.

A great extent of country between the Pyrenees and the Gironde is overspread by tertiary deposits of various ages, and chiefly of Miocene date. Some of these, near Bordeaux, coincide in age with the faluns of Touraine, already mentioned, but many of the species of sh.e.l.ls are peculiar to the south. The succession of beds in the basin of the Gironde implies several oscillations of level by which the same wide area was alternately converted into sea and land and into brackish-water lagoons, and finally into fresh-water ponds and lakes.

Among the fresh-water strata of this age near the base of the Pyrenees are marls, limestones and sands, in which the eminent comparative anatomist, M.

Lartet, has obtained a great number of fossil mammalia common to the faluns of the Loire and the Upper Miocene beds of Switzerland, such as Dinotherium giganteum and Mastodon angustidens; also the bones of quadrumana, or of the ape and monkey tribe, which were discovered in 1837, the first of that order of quadrupeds detected in Europe. They were found near Auch, in the Department of Gers, in lat.i.tude 43 degrees 39' N. About forty miles west of Toulouse. They were referred by MM. Lartet and Blainville to a genus closely allied to the Gibbon, to which they gave the name of Pliopithecus. Subsequently, in 1856, M.

Lartet described another species of the same family of long-armed apes (Hylobates), which he obtained from strata of the same age at Saint-Gaudens, in the Haute Garonne. The fossil remains of this animal consisted of a portion of a lower jaw with teeth and the shaft of a humerus. It is supposed to have been a tree-climbing frugivorous ape, equalling man in stature. As the trunks of oaks are common in the lignite beds in which it lay, it has received the generic name of Dryopithecus. The angle formed by the ascending ramus of the jaw and the alveolar border is less open, and therefore more like the human subject, than in the Chimpanzee, and what is still more remarkable, the fossil, a young but adult individual, had all its milk teeth replaced by the second set, while its last true molar (or wisdom-tooth) was still undeveloped, or only existed as a germ in the jaw-bone. In the mode, therefore, of the succession of its teeth (which, as in all the old-World apes, exactly agree in number with those in man) it differed from the Gorilla and Chimpanzee, and corresponded with the human species.

UPPER MIOCENE BEDS OF OENINGEN, IN SWITZERLAND.

The faluns of the Loire first served, as already stated, as the type of the Miocene formations in Europe. They yielded a plentiful harvest of marine fossil sh.e.l.ls and corals, but were entirely barren of plants and insects. In Switzerland, on the other hand, deposits of the same age have been discovered, remarkable for their botanical and entomological treasures. We are indebted to Professor Heer, of Zurich, for the description, restoration, and cla.s.sification of several hundred species and varieties of these fossil plants, the whole of which he has ill.u.s.trated by excellent figures in his "Flora Tertiaria Helvetiae." This great work, and those of Adolphe Brongniart, Unger, Goppert and others, show that this cla.s.s of fossils is beginning to play the same important part in the cla.s.sification of the tertiary strata containing lignite or brown coal as an older flora has long played in enabling us to understand the ancient coal or carboniferous formation. No small skepticism has always prevailed among botanists as to whether the leaves alone and the wood of plants could ever afford sufficient data for determining even genera and families in the vegetable kingdom. In truth, before such remains could be rendered available a new science had to be created. It was necessary to study the outlines, nervation, and microscopic structure of the leaves, with a degree of care which had never been called for in the cla.s.sification of living plants, where the flower and fruit afforded characters so much more definite and satisfactory. As geologists, we can not be too grateful to those who, instead of despairing when so difficult a task was presented to them, or being discouraged when men of the highest scientific attainments treated the fossil leaves as worthless, entered with full faith and enthusiasm into this new and unexplored field. That they should frequently have fallen into errors was unavoidable, but it is remarkable, especially if we inquire into the history of Professor Heer's researches, how often early conjectures as to the genus and family founded on the leaves alone were afterwards confirmed when fuller information was obtained. As examples to be found on comparing Heer's earlier and later works, I may instance the chestnut, elm, maple, cinnamon, magnolia, buckbean or Menyanthes, vine, buckthorn (Rhamnus), Andromeda and Myrica, and among the conifers Sequoia and Taxodium. In all these cases the plants were first recognised by their leaves, and the accuracy of the determination was afterwards confirmed when the fruit, and in some instances both fruit and flower, were found attached to the same stem as the leaves.