The Student's Elements of Geology Part 24

lignitum. Among the plants common to the Upper Miocene beds of Oeningen, in Switzerland, are Plata.n.u.s aceroides (Figure 141 Chapter 14), Myrica vindobonensis, and others.

UPPER MIOCENE STRATA OF ITALY.

We are indebted to Signor Michelotti for a valuable work on the Miocene sh.e.l.ls of Northern Italy. Those found in the hill called the Superga, near Turin, have long been known to correspond in age with the faluns of Touraine, and they contain so many species common to the Upper Miocene strata of Bordeaux as to lead to the conclusion that there was a free communication between the northern part of the Mediterranean and the Bay of Biscay in the Upper Miocene period.

UPPER MIOCENE FORMATIONS OF GREECE.

At Pikerme, near Athens, MM. Wagner and Roth have described a deposit in which they found the remains of the genera Mastodon, Dinotherium, Hipparion, two species of Giraffe, Antelope, and others, some living and some extinct. With them were also a.s.sociated fossil bones of the Semnopithecus, showing that here, as in the south of France, the quadrumana were characteristic of this period.

The whole fauna attests the former extension of a vast expanse of gra.s.sy plains where we have now the broken and mountainous country of Greece; plains, which were probably united with Asia Minor, spreading over the area where the deep Aegean Sea and its numerous islands are now situated. We are indebted to M.

Gaudry, who visited Pikerme, for a treatise on these fossil bones, showing how many data they contribute to the theory of a transition from the mammalia of the Upper Miocene through the Pliocene and Post-pliocene forms to those of living genera and species.

UPPER MIOCENE OF INDIA. SIWALIK HILLS.

The Siwalik Hills lie at the southern foot of the Himalayan chain, rising to the height of 2000 and 3000 feet. Between the Jumna and the Ganges they consist of inclined strata of sandstone, shingle, clay, and marl. We are indebted to the indefatigable researches of Dr. Falconer and Sir Proby Cautley, continued for fifteen years, for the discovery in these marls and sandstones of a great variety of fossil mammalia and reptiles, together with many fresh-water sh.e.l.ls.

Out of fifteen species of sh.e.l.ls of the genera Paludina, Melania, Ampullaria, and Unio, all are extinct or unknown species with the exception of four, which are still inhabitants of Indian rivers. Such a proportion of living to extinct mollusca agrees well with the usual character of an Upper Miocene or Falunian fauna, as observed in Touraine, or in the basin of Vienna and elsewhere.

The genera of mammalia point in the same direction. One of them, of the genus Chalicotherium (or Anisodon of Lartet), is a pachyderm intermediate between the Rhinoceros and Anoplothere, and characteristic of the Upper Miocene strata of Eppelsheim, and of the south of France. With it occurs also an extinct form of Hippopotamus, called Hexaprotodon, and a species of Hippotherium and pig, also two species of Mastodon, two of elephant, and three other elephantine proboscidians; none of them agreeing with any fossil forms of Europe, and being intermediate between the genera Elephas and Mastodon, const.i.tuting the sub-genus SteG.o.don of Falconer. With these are a.s.sociated a monkey, allied to the Semnopithecus entellus, now living in the Himalaya, and many ruminants. Among these last, besides the giraffe, camel, antelope, stag, and others, we find a remarkable new type, the Sivatherium, like a gigantic four-horned deer. There are also new forms of carnivora, both feline and canine, the Machairodus among the former, also hyaenas, and a subursine form called the Hyaenarctos, and a genus allied to the otter (Enhydriodon), of formidable size.

The giraffe, camel, and a large ostrich may be cited as proofs that there were formerly extensive plains where now a steep chain of hills, with deep ravines, runs for many hundred miles east and west. Among the accompanying reptiles are several crocodiles, some of huge dimensions, and one not distinguishable, says Dr. Falconer, from a species now living in the Ganges (C. Gangeticus); and there is still another saurian which the same anatomist has identified with a species now inhabiting India. There was also an extinct species of tortoise of gigantic proportions (Colossochelys Atlas), the curved sh.e.l.l of which was twelve feet three inches long and eight feet in diameter, the entire length of the animal being estimated at eighteen feet, and its probable height seven feet.

Numerous fossils of the Siwalik type have also been found in Perim Island, in the Gulf of Cambay, and among these a species of Dinotherium, a genus so characteristic of the Upper Miocene period in Europe.

OLDER PLIOCENE AND MIOCENE FORMATIONS IN THE UNITED STATES.

Between the Alleghany Mountains, formed of older rocks, and the Atlantic, there intervenes, in the United States, a low region occupied princ.i.p.ally by beds of marl, clay, and sand, consisting of the cretaceous and tertiary formations, and chiefly of the latter. The general elevation of this plain bordering the Atlantic does not exceed 100 feet, although it is sometimes several hundred feet high. Its width in the middle and southern states is very commonly from 100 to 150 miles. It consists, in the South, as in Georgia, Alabama, and South Carolina, almost exclusively of Eocene deposits; but in North Carolina, Maryland, Virginia, Delaware, more modern strata predominate, of the age of the English Crag and faluns of Touraine. (Proceedings of the Geological Society volume 4 part 3 1845 page 547.)

(FIGURE 148. Fulgur ca.n.a.liculatus. Maryland.)

(FIGURE 149. Fusus quadricostatus, Say. Maryland.)

In the Virginian sands, we find in great abundance a species of Astarte (A.

undulata, Conrad), which resembles closely, and may possibly be a variety of, one of the commonest fossils of the Suffolk Crag (A. Omalii); the other sh.e.l.ls also, of the genera Natica, Fissurella, Artemis, Lucina, Chama, Pectunculus, and Pecten, are a.n.a.lagous to sh.e.l.ls both of the English Crag and French faluns, although the species are almost all distinct. Out of 147 of these American fossils I could only find thirteen species common to Europe, and these occur partly in the Suffolk Crag, and partly in the faluns of Touraine; but it is an important characteristic of the American group, that it not only contains many peculiar extinct forms, such as Fusus quadricostatus, Say (see Figure 149), and Venus tridacnoides, abundant in these same formations, but also some sh.e.l.ls which, like Fulgur carica of Say and F. ca.n.a.liculatus (see Figure 148), Calyptraea costata, Venus mercenaria, Lam., Modiola glandula, Totten, and Pecten magellanicus, Lam., are recent species, yet of forms now confined to the western side of the Atlantic-- a fact implying that some traces of the beginning of the present geographical distribution of mollusca date back to a period as remote as that of the Miocene strata.

Of ten species of corals which I procured on the banks of the James River, one agrees generically with a coral now living on the coast of the United States.

Mr. Lonsdale regarded these corals as indicating a temperature exceeding that of the Mediterranean, and the sh.e.l.ls would lead to similar conclusions. Those occurring on the James River are in the 37th degree of N. lat.i.tude, while the French faluns are in the 47th; yet the forms of the American fossils would scarcely imply so warm a climate as must have prevailed in France when the Miocene strata of Touraine originated.

(FIGURE 150. Astrangia lineata, Lonsdale. Syn. Anthophyllum lineatum.

Williamsburg, Virginia.)

Among the remains of fish in these post-eocene strata of the United States are several large teeth of the shark family, not distinguishable specifically from fossils of the faluns of Touraine.

CHAPTER XV.

LOWER MIOCENE (OLIGOCENE OF BEYRICH).

Lower Miocene Strata of France.

Line between Miocene and Eocene.

Lacustrine Strata of Auvergne.

Fossil mammalia of the Limagne d'Auvergne.

Lower Mola.s.se of Switzerland.

Dense Conglomerates and Proofs of Subsidence.

Flora of the Lower Mola.s.se.

American Character of the Flora.

Theory of a Miocene Atlantis.

Lower Miocene of Belgium.

Rupelian Clay of Hermsdorf near Berlin.

Mayence Basin.

Lower Miocene of Croatia.

Oligocene Strata of Beyrich.

Lower Miocene of Italy.

Lower Miocene of England.

Hempstead Beds.

Bovey Tracey Lignites in Devonshire.

Isle of Mull Leaf-Beds.

Arctic Miocene Flora.

Disco Island.

Lower Miocene of United States.

Fossils of Nebraska.

LINE BETWEEN MIOCENE AND EOCENE FORMATIONS.

The marine faluns of the valley of the Loire have been already described as resting in some places on a fresh-water tertiary limestone, fragments of which have been broken off and rolled on the sh.o.r.es and in the bed of the Miocene sea.

Such pebbles are frequent at Pontlevoy on the Cher, with hollows drilled in them in which the perforating marine sh.e.l.ls of the Falunian period still remain. Such a mode of superposition implies an interval of time between the origin of the fresh-water limestone and its submergence beneath the waters of the Upper Miocene sea. The limestone in question forms a part of the formation called the Calcaire de la Beauce, which const.i.tutes a large table-land between the basins of the Loire and the Seine. It is a.s.sociated with marls and other deposits, such as may have been formed in marshes and shallow lakes in the newest part of a great delta. Beds of flint, continuous or in nodules, acc.u.mulated in these lakes, and aquatic plants called Charae, left their stems and seed-vessels imbedded both in the marl and flint, together with fresh-water and land sh.e.l.ls.

Some of the siliceous rocks of this formation are used extensively for mill- stones. The flat summits or platforms of the hills round Paris, and large areas in the forest of Fontainebleau, as well as the Plateau de la Beauce, already alluded to, are chiefly composed of these fresh-water strata. Next to these in the descending order are marine sands and sandstone, commonly called the Gres de Fontainebleau, from which a considerable number of sh.e.l.ls, very distinct from those of the faluns, have been obtained at Etampes, south of Paris, and at Montmartre and other hills in Paris itself, or in its suburbs. At the bottom of these sands a green clay occurs, containing a small oyster, Ostrea cyathula, Lam., which, although of slight thickness, is spread over a wide area. This clay rests immediately on the Paris gypsum, or that series of beds of gypsum and gypseous marl from which Cuvier first obtained several species of Palaeotherium and other extinct mammalia. (Bulletin 1856 Journal volume 12 page 768.)

At this junction of the clay and the gypsum the majority of French geologists have always drawn the line between the Middle and Lower Tertiary, or between the Miocene and Eocene formations, regarding the Fontainebleau sands and the Ostrea cyathula clay as the base of the Miocene, and the gypsum, with its mammalia, as the top of the Eocene group. I formerly dissented from this division, but I now find that I must admit it to be the only one which will agree with the distribution of the Miocene mammalia, while even the mollusca of the Fontainebleau sands, which were formerly supposed to present at preponderance of affinities to an Eocene fauna, have since been shown to agree more closely with the fossils of certain deposits always regarded as Middle Tertiary at Mayence and in Belgium. In fact, we are now arriving at that stage of progress when the line, wherever it be drawn between Miocene and Eocene, will be an arbitrary one, or one of mere convenience, as I shall have an opportunity of showing when the Upper Eocene formations in the Isle of Wight are described in the sixteenth chapter.

LOWER MIOCENE OF CENTRAL FRANCE.

Lacustrine strata, belonging, for the most part, to the same Miocene system as the Calcaire de la Beauce, are again met with farther south in Auvergne, Cantal, and Velay. They appear to be the monuments of ancient lakes, which, like some of those now existing in Switzerland, once occupied the depressions in a mountainous region, and have been each fed by one or more rivers and torrents.

The country where they occur is almost entirely composed of granite and different varieties of granitic schist, with here and there a few patches of Secondary strata, much dislocated, and which have suffered great denudation.

There are also some vast piles of volcanic matter, the greater part of which is newer than the fresh-water strata, and is sometimes seen to rest upon them, while a small part has evidently been of contemporaneous origin. Of these igneous rocks I shall treat more particularly in the sequel.

The study of these regions possesses a peculiar interest very distinct in kind from that derivable from the investigation either of the Parisian or English Tertiary areas. For we are presented in Auvergne with the evidence of a series of events of astonishing magnitude and grandeur, by which the original form and features of the country have been greatly changed, yet never so far obliterated but that they may still, in part at least, be restored in imagination. Great lakes have disappeared-- lofty mountains have been formed, by the reiterated emission of lava, preceded and followed by showers of sand and scoriae-- deep valleys have been subsequently furrowed out through ma.s.ses of lacustrine and volcanic origin-- at a still later date, new cones have been thrown up in these valleys-- new lakes have been formed by the damming up of rivers-- and more than one a.s.semblage of quadrupeds, birds, and plants, Eocene, Miocene, and Pliocene, have followed in succession; yet the region has preserved from first to last its geographical ident.i.ty; and we can still recall to our thoughts its external condition and physical structure before these wonderful vicissitudes began, or while a part only of the whole had been completed. There was first a period when the s.p.a.cious lakes, of which we still may trace the boundaries, lay at the foot of mountains of moderate elevation, unbroken by the bold peaks and precipices of Mont Dor, and unadorned by the picturesque outline of the Puy de Dome, or of the volcanic cones and craters now covering the granitic platform. During this earlier scene of repose deltas were slowly formed; beds of marl and sand, several hundred feet thick, deposited; siliceous and calcareous rocks precipitated from the waters of mineral springs; sh.e.l.ls and insects imbedded, together with the remains of the crocodile and tortoise, the eggs and bones of water-birds, and the skeletons of quadrupeds, most of them of genera and species characteristic of the Miocene period. To this tranquil condition of the surface succeeded the era of volcanic eruptions, when the lakes were drained, and when the fertility of the mountainous district was probably enhanced by the igneous matter ejected from below, and poured down upon the more sterile granite. During these eruptions, which appear to have taken place towards the close of the Miocene epoch, and which continued during the Pliocene, various a.s.semblages of quadrupeds successively inhabited the district, among which are found the genera mastodon, rhinoceros, elephant, tapir, hippopotamus, together with the ox, various kinds of deer, the bear, hyaena, and many beasts of prey which ranged the forest or pastured on the plain, and were occasionally overtaken by a fall of burning cinders, or buried in flows of mud, such as accompany volcanic eruptions. Lastly, these quadrupeds became extinct, and gave place in their turn to the species now existing. There are no signs, during the whole time required for this series of events, of the sea having intervened, nor of any denudation which may not have been accomplished by currents in the different lakes, or by rivers and floods accompanying repeated earthquakes, or subterranean movements, during which the levels of the district have in some places been materially modified, and perhaps the whole upraised relatively to the surrounding parts of France.

AUVERGNE.

The most northern of the fresh-water groups is situated in the valley-plain of the Allier, which lies within the department of the Puy de Dome, being the tract which went formerly by the name of the Limagne d'Auvergne. The average breadth of this tract is about twenty miles; and it is for the most part composed of nearly horizontal strata of sand, sandstone, calcareous marl, clay, and limestone, none of which observe a fixed and invariable order of superposition.

The ancient borders of the lake wherein the fresh-water strata were acc.u.mulated may generally be traced with precision, the granite and other ancient rocks rising up boldly from the level country. The actual junction, however, of the lacustrine beds and the granite is rarely seen, as a small valley usually intervenes between them. The fresh-water strata may sometimes be seen to retain their horizontality within a very slight distance of the border-rocks, while in some places they are inclined, and in few instances vertical. The princ.i.p.al divisions into which the lacustrine series may be separated are the following:-- first, Sandstone, grit, and conglomerate, including red marl and red sandstone; secondly, Green and white foliated marls; thirdly, Limestone, or travertin, often oolitic in structure; fourthly, Gypseous marls.

The relations of these different groups can not be learnt by the study of any one section; and the geologist who sets out with the expectation of finding a fixed order of succession may perhaps complain that the different parts of the basin give contradictory results. The arenaceous division, the marls, and the limestone may all be seen in some places to alternate with each other; yet it can by no means be affirmed that there is no order of arrangement. The sands, sandstone, and conglomerate const.i.tute in general a littoral group; the foliated white and green marl, a contemporaneous central deposit more than 700 feet thick, and thinly foliated, a character which often arises from the innumerable thin sh.e.l.ls or carapace valves shed by the small crustacean called Cypris in the ancient lakes of Auvergne; and lastly the limestone is for the most part subordinate to the newer portions of both the above formations.

It seems that, when the ancient lake of the Limagne first began to be filled with sediment, no volcanic action had yet produced lava and scoriae on any part of the surface of Auvergne. No pebbles, therefore, of lava were transported into the lake-- no fragments of volcanic rocks imbedded in the conglomerate. But at a later period, when a considerable thickness of sandstone and marl had acc.u.mulated, eruptions broke out, and lava and tuff were deposited, at some spots, alternately with the lacustrine strata. It is not improbable that cold and thermal springs, holding different mineral ingredients in solution, became more numerous during the successive convulsions attending this development of volcanic agency, and thus deposits of carbonate and sulphate of lime, silex, and other minerals were produced. Hence these minerals predominate in the uppermost strata. The subterranean movements may then have continued until they altered the relative levels of the country, and caused the waters of the lakes to be drained off, and the further acc.u.mulation of regular fresh-water strata to cease.

LOWER MIOCENE MAMMALIA OF THE LIMAGNE.

It is scarcely possible to determine the age of the oldest part of the fresh- water series of the Limagne, large ma.s.ses both of the sandy and marly strata being devoid of fossils. Some of the lowest beds may be of Upper Eocene date, although, according to M. Pomel, only one bone of a Palaeotherium has been discovered in Auvergne. But in Velay, in strata containing some species of fossil mammalia common to the Limagne, no less than four species of Palaeothere have been found by M. Aymard, and one of these is generally supposed to be identical with Palaeotherium magnum, an undoubted Upper Eocene fossil, of the Paris gypsum, the other three being peculiar.