Part 23 (1/2)
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.