A Manual of Elementary Geology Part 23

28.), now inhabiting the Loire; and _Cyrena consobrina_ (fig. 26. p.

28.). The last-mentioned fossil (a recent Egyptian sh.e.l.l of the Nile) is very abundant at Grays, and deserves notice, because the genus _Cyrena_ is now no longer European.

The rhinoceros occurring in the same beds (_R. leptorhinus_, see fig. 131.

p. 160.) is of a different species from that of Brentford above mentioned, and the accompanying elephant belongs to the variety called _Elephas meridionalis_, which, according to MM. Owen and H. von Meyer, two high authorities, is the same species as the Siberian mammoth, although some naturalists regard it as distinct. With the above mammalia is also found the _Hippopotamus major_, and what is most remarkable in so modern and northern a deposit, a monkey, called by Owen, _Macacus pliocenus_.

The submerged forest already alluded to (p. 130.) as underlying the drift at the base of the cliffs of Norfolk is a.s.sociated with a bed of lignite and loam, in which a great number of fossil bones occur, apparently of the same group as that of Grays, just mentioned. It has sometimes been called "the Elephant bed." One portion of it, which stretches out under the sea at Happisburgh, was overgrown in 1820 by a bank of recent oysters, and there the fishermen dredged up, according to Woodward, in the course of thirteen years, together with the oysters, above 2000 mammoths' grinders.[147-B]

Another portion of the same continuous stratum has yielded at Bacton, Cromer, and other places on the coast, the bones of a gigantic beaver (_Trogontherium Cuvierii_, Fischer), as well as the ox, horse, and deer, and both species of rhinoceros, _R. tichorhinus_ and _R. leptorhinus_.

In studying these and various other similar a.s.semblages of fossils, we have a good exemplification of the more rapid rate at which the mammiferous fauna, as compared to the testaceous, diverges when traced backwards in time from the recent type. I have before hinted, that the longevity of species in the cla.s.s of warm-blooded quadrupeds is less great than in that of the mollusca, the latter having probably more capacity for enduring those changes of climate and other external circ.u.mstances which take place in the course of ages on the earth's surface. This phenomenon is by no means confined to Europe, for Mr. Darwin found at Bahia Blanca, in South America, lat. 39 S., near the northern confines of Patagonia, fossil remains of the extinct mammiferous genera Megatherium, Megalonyx, Toxodon, and others, a.s.sociated with sh.e.l.ls, almost all of species already ascertained to be still living in the contiguous sea[148-A]; the marine mollusca, as well as those of rivers, lakes, or the land, having died out more slowly than the terrestrial mammalia.

I alluded before (p. 125.) to certain marine strata overlying till near Glasgow, and at other points on the Clyde, in which the sh.e.l.ls are for the most part British, with an intermixture of some arctic species; while others, about a tenth of the whole, are supposed to be extinct. This formation may also be called Newer Pliocene.

_Fluvio-marine crag of Norwich._--At several places within five miles of Norwich, on both banks of the Yare, beds of sand, loam, and gravel, provincially termed "crag," occur, in which there is a mixture of marine, land, and freshwater sh.e.l.ls, with ichthyolites and bones of mammalia. It is clear that these beds have been acc.u.mulated at the bottom of the sea near the mouth of a river. They form patches of variable thickness, resting on white chalk, and are covered by a dense ma.s.s of stratified flint gravel.

The surface of the chalk is often perforated to the depth of several inches by the _Pholas crispata_, each fossil sh.e.l.l still remaining at the bottom of its cylindrical cavity, now filled up with loose sand which has fallen from the inc.u.mbent crag. This species of Pholas still exists and drills the rocks between high and low water on the British coast. The most common sh.e.l.ls of these strata, such as _Fusus striatus_, _Turritella terebra_, _Cardium edule_, and _Cyprina islandica_, are now abundant in the British seas; but with them are some extinct species, such as _Nucula Cobboldiae_ (fig. 120.) and _Tellina obliqua_ (fig. 121.). _Natica helicoides_ (fig.

122.) is an example of a species formerly known only as fossil, but which has now been found living in our seas.

Among the accompanying bones of mammalia is the _Mastodon_ _angustidens_[149-A] (see fig. 130.), a portion of the upper jawbone with a tooth having been found by Mr. Wigham at Postwick, near Norwich. As this species has also been found in the Red Crag, both at Sutton and at Felixstow, and had hitherto been regarded as characteristic of formations older than the Pleistocene, it may possibly have been washed out of the Red into the Norwich Crag.

[Ill.u.s.tration: Fig. 120. _Nucula Cobboldiae._]

[Ill.u.s.tration: Fig. 121. _Tellina obliqua._]

[Ill.u.s.tration: Fig. 122. _Natica helicoides_, Johnston.]

Among the bones, however, respecting the authenticity of which there seems no doubt, may be mentioned those of the elephant, horse, pig, deer, and the jaws and teeth of field mice (fig. 141.). I have seen the tusk of an elephant from Bramerton near Norwich, to which, many serpulae were attached, showing that it had lain for some time at the bottom of the sea of the Norwich Crag.

At Thorpe, near Aldborough, and at Southwold, in Suffolk, this fluvio-marine formation is well exposed in the sea-cliffs, consisting of sand, s.h.i.+ngle, loam, and laminated clay. Some of the strata there bear the marks of tranquil deposition, and in one section a thickness of 40 feet is sometimes exposed to view. Some of the lamellibranchiate sh.e.l.ls have both valves united, although mixed with land and freshwater testacea, and with the bones and teeth of elephant, rhinoceros, horse, and deer. Captain Alexander, with whom I examined these strata in 1835, showed me a bed rich in marine sh.e.l.ls, in which he had found a large specimen of the _Fusus striatus_, filled with sand, and in the interior of which was the tooth of a horse.

Among the freshwater sh.e.l.ls I obtained the _Cyrena consobrina_ (fig. 26.

p. 28.), before mentioned, supposed to agree with a species now living in the Nile.

I formerly cla.s.sed the Norwich Crag as older Pliocene, conceiving that more than a third of the fossil testacea were extinct; but there now seems good reason for believing that several of the rarer sh.e.l.ls obtained from these strata do not really belong to a contemporary fauna, but have been washed out of the older beds of the "Red Crag;" while other species, once supposed to have died out, have lately been met with living in the British seas.

According to Mr. Searles Wood, the total number of marine species does not exceed seventy-six, of which one tenth only are extinct. Of the fourteen a.s.sociated freshwater sh.e.l.ls, all the species appear to be living. Strata containing the same sh.e.l.ls as those near Norwich have been found by Mr.

Bean, at Bridlington, in Yorks.h.i.+re.

_Newer Pliocene strata of Sicily._--In no part of Europe are the Newer Pliocene formations seen to enter so largely into the structure of the earth's crust, or to rise to such heights above the level of the sea, as in Sicily. They cover nearly half the island, and near its centre, at Castrogiovanni, they reach an elevation of 3000 feet. They consist princ.i.p.ally of two divisions, the upper calcareous, 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. Of the remainder, which still live, five species are no longer inhabitants of the Mediterranean. When I visited Sicily in 1828 I estimated the proportion of living species as somewhat greater, partly because I confounded with the tertiary formation of central Sicily the strata at the base of Etna, and some other localities, where the fossils are now proved to agree entirely with the present Mediterranean fauna.

Philippi came to the conclusion, that in Sicily there is a gradual pa.s.sage from beds containing 70 per cent. of recent sh.e.l.ls, to those in which the whole of the fossils are identical with recent species; but his tables appear scarcely to bear out so important a generalization, several of the places cited by him in confirmation having as yet furnished no more than twenty or thirty species of testacea. The Sicilian beds in question probably belong to about the same period as the Norwich Crag, although a geologist, accustomed to see nearly all the Pleistocene 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 downwards into a sandstone and conglomerate, below which is clay and blue marl, like that of the Subapennine hills, 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, volcanos 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_.

In part of the region above alluded to, as, for example, near Lentini, a conglomerate occurs in which I observed many pebbles of volcanic rocks covered by full grown _serpulae_. We may explain the origin of these by supposing that there were some small volcanic islands which may have been destroyed from time to time by the waves, as Graham Island has been swept away since 1831. The rounded blocks and pebbles of solid volcanic matter, after being rolled for a time on the beach of such temporary islands, were carried at length into some tranquil part of the sea, where they lay for years, while the marine _serpulae_ adhered to them, their sh.e.l.ls growing and covering their surface, as they are seen adhering to the sh.e.l.l figured in p. 22. Finally, the bed of pebbles was itself covered with strata of sh.e.l.ly limestone. At Vizzini, a town not many miles distant to the S.W., I remarked another striking proof of the gradual manner in which these modern rocks were formed, and the long intervals of time which elapsed between the pouring out of distinct sheets of lava. A bed of oysters no less than 20 feet in thickness rests upon a current of basaltic lava. The oysters are perfectly identifiable with our common eatable species. Upon the oyster bed, again, is superimposed a second ma.s.s of lava, together with tuff or peperino. In the midst of the same alternating igneous and aqueous formations is seen near Galieri, not far from Vizzini, a horizontal bed, about a foot and a half in thickness, composed entirely of a common Mediterranean coral (_Caryophyllia caespitosa_, Lam.). These corals stand erect as they grew; and, after being traced for hundreds of yards, are again found at a corresponding height on the opposite side of the valley.

[Ill.u.s.tration: Fig. 123. _Caryophyllia caespitosa_, Lam.

(_Cladocora caespitosa_, Ehr.)

_a._ Stem with young stem growing from its side.

_a*._ Young stem of same twice magnified.

_b._ Portion of branch, twice magnified, with the base of a lateral branch; the exterior ridges of the main branch appearing through the lamellae of the lateral one.

_c._ Transverse section of same, proving, by the integrity of the main branch, that the lateral one did not originate in a subdivision of the animal.

_d._ A branch, having at its base another laterally united to it, and two young corals at its upper part.

_e._ A main branch, with a full grown lateral one.

_f._ A perfect terminal star.]

The corals are usually branched, but not by the division of the animals as some have supposed, but by the attachment of young individuals to the sides of the older ones; and we must understand this mode of increase, in order to appreciate the time which was required for the building up of the whole bed of coral during the growth of many successive generations.[152-A]

Among the other fossil sh.e.l.ls met with in these Sicilian strata, which still continue to abound in the Mediterranean, no sh.e.l.l is more conspicuous, from its size and frequent occurrence, than the great scallop, _Pecten jacobaeus_ (see fig. 124.), now so common in the neighbouring seas. We see this sh.e.l.l in the calcareous beds at Palermo in great numbers, in the limestone at Girgenti, and in that which alternates with volcanic rocks in the country between Syracuse and Vizzini, often at great heights above the sea.

[Ill.u.s.tration: Fig. 124. _Pecten jacobaeus_; half natural size.]

The more we reflect on the preponderating number of these 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 appearance unaltered since the Greeks first colonised the island.

The modern geological date of the rocks in this region leads to another singular and unexpected conclusion, namely, that the fauna and flora of a large part of Sicily are of higher antiquity than the country itself, having not only flourished before the lands were raised from the deep, but even before their materials were brought together beneath the waters. The chain of reasoning which conducts us to this opinion may be stated in a few words. The larger part of the island has been converted from sea into land since the Mediterranean was peopled with nearly all the living species of testacea and zoophytes. We may therefore presume that, before this region emerged, the same land and river sh.e.l.ls, and almost all the same animals and plants, were in existence which now people Sicily; for the terrestrial fauna and flora of this island are precisely the same as that of other lands surrounding the Mediterranean. There appear to be no peculiar or indigenous species, and those which are now established there must be supposed to have migrated from pre-existing lands, just as the plants and animals of the Neapolitan territory have colonised Monte Nuovo, since that volcanic cone was thrown up in the sixteenth century.

Such conclusions throw a new light on the adaptation of the attributes and migratory habits of animals and plants to the changes which are unceasingly in progress in the physical geography of the globe. It is clear that the duration of species is so great, that they are destined to outlive many important revolutions in the configuration of the earth's surface; and hence those innumerable contrivances for enabling the subjects of the animal and vegetable creation to extend their range; the inhabitants of the land being often carried across the ocean, and the aquatic tribes over great continental s.p.a.ces. It is obviously expedient that the terrestrial and fluviatile species should not only be fitted for the rivers, valleys, plains, and mountains which exist at the era of their creation, but for others that are destined to be formed before the species shall become extinct; and, in like manner, the marine species are not only made for the deep and shallow regions of the ocean existing at the time when they are called into being, but for tracts that may be submerged or variously altered in depth during the time that is allotted for their continuance on the globe.

OSSEOUS BRECCIAS AND DEPOSITS IN CAVES OF THE PLIOCENE PERIOD.

_Sicily._--Caverns filled with marine breccias, at the base of ancient sea-cliffs, have been already mentioned in the sixth chapter; and it was noticed, respecting the cave of San Ciro, near Palermo (p. 75.), that upon a bed of sand filled with sea-sh.e.l.ls, almost all of recent species, rests a breccia (_b_, fig. 93.), composed of fragments of calcareous rock, and the bones of animals. In the sand at the bottom of that cave, Dr. Philippi found about forty-five marine sh.e.l.ls, all clearly identical with recent species, except two or three. The bones in the inc.u.mbent breccia are chiefly those of the mammoth (_E. primigenius_), with some belonging to an hippopotamus, distinct from the recent species, and smaller than that usually found fossil. (See fig. 132.) Several species of deer also, and, according to some accounts, the remains of a bear, were discovered. These mammalia are probably referable to the Post-Pliocene period.

The Newer Pliocene tertiary limestone of the south of Sicily, already described, is sometimes full of caverns; and the student will at once perceive that all the quadrupeds of which the remains are found in the stalact.i.te of these caverns, being of later origin than the rocks, must be referable to the close of the tertiary epoch, if not of still later date.

The situation of one of these caves, in the valley of Sortino, is represented in the annexed section.

[Ill.u.s.tration: Fig. 125. Cross section.

_a_. Alluvium, } containing the remains of quadrupeds _b_, _b_. Deposits in caves, } for the most part extinct.

C. Limestone, containing the remains of sh.e.l.ls, of which between 70 and 80 per cent. are recent.]

_England._--In a cave at Kirkdale, about twenty-five miles N.N.E. of York, the remains of about 300 hyaenas, belonging to individuals of every age, have been detected. The species (_Hyaena spelaea_) is extinct, and was larger than the fierce _Hyaena crocuta_ of South Africa, which it most resembled.

Dr. Buckland, after carefully examining the spot, proved that the Hyaenas must have lived there; a fact attested by the quant.i.ty of their dung, which, as in the case of the living hyaena, is of nearly the same composition as bone, and almost as durable. In the cave were found the remains of the ox, young elephant, hippopotamus, rhinoceros, horse, bear, wolf, hare, water-rat, and several birds. All the bones have the appearance of having been broken and gnawed by the teeth of the hyaenas; and they occur confusedly mixed in loam or mud, or dispersed through a crust of stalagmite which covers it. In these and many other cases it is supposed that portions of herbivorous quadrupeds have been dragged into caverns by beasts of prey, and have served as their food, an opinion quite consistent with the known habits of the living hyaena.

No less than thirty-seven species of mammalia are enumerated by Professor Owen as having been discovered in the caves of the British islands, of which eighteen appear to be extinct, while the others still survive in Europe. They were not washed to the spots where the fossils now occur by a great flood; but lived and died, one generation after another, in the places where they lie buried. Among other arguments in favour of this conclusion may be mentioned the great numbers of the shed antlers of deer discovered in caves and in freshwater strata throughout England.[155-A]

Examples also occur of fissures into which animals have fallen from time to time, or have been washed in from above, together with alluvial matter and fragments of rock detached by frost, forming a ma.s.s which may be united into a bony breccia by stalagmitic infiltrations. Frequently we discover a long suite of caverns connected by narrow and irregular galleries, which hold a tortuous course through the interior of mountains, and seem to have served as the subterranean channels of springs and engulphed rivers. Many streams in the Morea are now carrying bones, pebbles, and mud into underground pa.s.sages of this kind.[155-B] If, at some future period, the form of that country should be wholly altered by subterranean movements and new valleys shaped out by denudation, many portions of the former channels of these engulphed streams may communicate with the surface, and become the dens of wild beasts, or the recesses to which quadrupeds retreat to die.

Certain caves of France, Germany, and Belgium, may have pa.s.sed successively through these different conditions, and in their last state may have remained open to the day for several tertiary periods. It is nevertheless remarkable, that on the continent of Europe, as in England, the fossil remains of mammalia belong almost exclusively to those of the Newer Pliocene and Post-Pliocene periods, and not to the Miocene or Eocene epochs, and when they are accompanied by land or river sh.e.l.ls, these agree in great part, or entirely, with recent species.

As the preservation of the fossil bones is due to a slow and constant supply of stalact.i.te, brought into the caverns by water dropping from the roof, the source and origin of this deposit has been a subject of curious inquiry. The following explanation of the phenomenon has been recently suggested by the eminent chemist Liebig. On the surface of Franconia, where the limestone abounds in caverns, is a fertile soil, in which vegetable matter is continually decaying. This mould or humus, being acted on by moisture and air, evolves carbonic acid which is dissolved by rain. The rain water, thus impregnated, permeates the porous limestone, dissolves a portion of it, and afterwards, when the excess of carbonic acid evaporates in the caverns, parts with the calcareous matter, and forms stalact.i.te.

_Australian cave-breccias._--Ossiferous breccias are not confined to Europe, but occur in all parts of the globe; and those lately discovered in fissures and caverns in Australia correspond closely in character with what has been called the bony breccia of the Mediterranean, in which the fragments of bone and rock are firmly bound together by a red ochreous cement.

Some of these caves have been examined by Sir T. Mitch.e.l.l in the Wellington Valley, about 210 miles west of Sidney, on the river Bell, one of the princ.i.p.al sources of the Macquarie, and on the Macquarie itself. The caverns often branch off in different directions through the rock, widening and contracting their dimensions, and the roofs and floors are covered with stalact.i.te. The bones are often broken, but do not seem to be water-worn.