A Manual of Elementary Geology Part 25

[148-A] Zool. of Beagle, part 1. pp. 9. 111.

[149-A] Owen, Brit. Foss. Mamm. 271. _Mastodon longirostris_, Kaup, see _ibid._

[152-A] I am indebted to Mr. Lonsdale for the details above given respecting the structure of this coral.

[155-A] Owen, Brit. Foss. Mam. xxvi., and Buckland, Rel. Dil. 19. 24.

[155-B] See Principles of Geology.

[158-A] See Principles of Geology, chaps. xli. to xliv.

CHAPTER XIV.

OLDER PLIOCENE AND MIOCENE FORMATIONS.

Strata of Suffolk termed Red and Coralline crag--Fossils, and proportion of recent species--Depth of sea and climate--Reference of Suffolk crag to the older Pliocene period--Migration of many species of sh.e.l.ls southwards during the glacial period--Fossil whales--Sub- apennine beds--Asti, Sienna, Rome--Miocene formations--Faluns of Touraine--Depth of sea and littoral character of fauna--Tropical climate implied by the testacea--Proportion of recent species of sh.e.l.ls--Faluns more ancient than the Suffolk crag--Miocene strata of Bordeaux and Piedmont--Mola.s.se of Switzerland--Tertiary strata of Lisbon--Older Pliocene and Miocene formations in the United States--Sewalik Hills in India.

The older Pliocene strata, which next claim our attention, are chiefly confined, in Great Britain, to the eastern part of the county of Suffolk, where, like the Norwich beds already described, they are called "Crag," a provincial name given particularly to those ma.s.ses of sh.e.l.ly sand which have been used from very ancient times in agriculture, to fertilize soils deficient in calcareous matter. The relative position of the "red crag" in Ess.e.x to the London clay, may be understood by reference to the accompanying diagram (fig. 142.).

[Ill.u.s.tration: Fig. 142. Cross section.]

These deposits, judging by the sh.e.l.ls which they contain, appear, according to Professor Edward Forbes, to have been formed in a sea of moderate depth, generally from 15 to 25 fathoms deep, although in some few spots perhaps deeper. But they may, nevertheless, have been acc.u.mulated at the distance of 40 or 50 miles from land.

The Suffolk crag is divisible into two ma.s.ses, the upper of which has been termed the Red, and the lower the Coralline Crag.[162-A] The upper deposit consists chiefly of quartzose sand, with an occasional intermixture of sh.e.l.ls, for the most part rolled, and sometimes comminuted. The lower or Coralline crag is of very limited extent, ranging over an area about 20 miles in length, and 3 or 4 in breadth, between the rivers Alde and Stour.

It is generally calcareous and marly--a ma.s.s of sh.e.l.ls and small corals, pa.s.sing occasionally into a soft building stone. At Sudbourn, near Orford, where it a.s.sumes this character, are large quarries, in which the bottom of it has not been reached at the depth of 50 feet. At some places in the neighbourhood, the softer ma.s.s is divided by thin flags of hard limestone, and corals placed in the upright position in which they grew.

The Red crag is distinguished by the deep ferruginous or ochreous colour of its sands and fossils, the Coralline by its white colour. Both formations are of moderate thickness; the red crag rarely exceeding 40, and the coralline seldom amounting to 20, feet. But their importance is not to be estimated by the density of the ma.s.s of strata or its geographical extent, but by the extraordinary richness of its organic remains, belonging to a very peculiar type, which seems to characterize the state of the living creation in the north of Europe during the older Pliocene era.

For a large collection of the fish, echinoderms, sh.e.l.ls, and corals of the deposits in Suffolk, we are indebted to the labours of Mr. Searles Wood. Of testacea alone he has obtained from 230 species from the Red, and 345 from the Coralline crag, about 150 being common to each. The proportion of recent species in the new group is considered by Mr. Wood to be about 70[162-B] per cent., and that in the older or coralline about 60. When I examined these sh.e.l.ls of Suffolk in 1835, with the a.s.sistance of Dr. Beck, Mr. George Sowerby, Mr. Searles Wood, and other eminent conchologists, I came to the opinion that the extinct species predominated very decidedly in number over the living. Recent investigations, however, have thrown much new light on the conchology of the Arctic, Scandinavian, British, and Mediterranean Seas. Many of the species formerly known only as fossils of the Crag, and supposed to have died out, have been dredged up in a living state from depths not previously explored. Other recent species, before regarded as distinct from the nearest allied Crag fossils, have been observed, when numerous individuals were procured, to be liable to much greater variation, both in size and form, than had been suspected, and thus have been identified. Consequently, the Crag fauna has been found to approach much more nearly to the recent fauna of the Northern, British, and Mediterranean Seas than had been imagined. The a.n.a.logy of the whole group of testacea to the European type is very marked, whether we refer to the large development of certain genera in number of species or to their size, or to the suppression or feeble representation of others. The indication also afforded by the entire fauna of a climate not much warmer than that now prevailing in corresponding lat.i.tudes, prepares us to believe that they are not of higher antiquity than the Older Pliocene era.[163-A]

[Ill.u.s.tration: Fig. 143. Section near Ipswich, in Suffolk.

_a._ Red crag.

_b._ Coralline crag.

_c._ London clay.]

The position of the red crag in Ess.e.x to the subjacent London clay and chalk has been already pointed out (fig. 142.). Whenever the two divisions are met with in the same district, the red crag lies uppermost; and, in some cases, as in the section represented in fig.

143., it is observed that the older or coralline ma.s.s _b_ had suffered denudation before the newer formation _a_ was thrown down upon it. At D there is not only a distinct cliff, 8 or 10 feet high, of coralline crag, running in a direction N.E. and S.W., against which the red crag abuts with its horizontal layers; but this cliff occasionally overhangs.

The rock composing it is drilled everywhere by _Pholades_, the holes which they perforated having been afterwards filled with sand and covered over when the newer beds were thrown down. As the older formation is shown by its fossils to have acc.u.mulated in a deeper sea (15, and sometimes 25, fathoms deep or more), there must no doubt have been an upheaval of the sea-bottom before the cliff here alluded to was shaped out. We may also conclude that so great an amount of denudation could scarcely take place, in such incoherent materials, without many of the fossils of the inferior beds becoming mixed up with the overlying crag, so that considerable difficulty must be occasionally experienced by the palaeontologist in deciding which species belong severally to each group. The red crag being formed in a shallower sea, often resembles in structure a s.h.i.+fting sand bank, its layers being inclined diagonally, and the planes of stratification being sometimes directed in the same quarry to the four cardinal points of the compa.s.s, as at Butley. That in this and many other localities, such a structure is not deceptive or due to any subsequent concretionary re-arrangement of particles, or to mere lines of colour, is proved by each bed being made up of flat pieces of sh.e.l.l which lie parallel to the planes of the smaller strata.

Some fossils, which are very abundant in the red crag, have never been found in the white or coralline division; as, for example, the _Fusus contrarius_ (fig. 144.), and several species of _Buccinum_ (or _Na.s.sa_) and _Murex_ (see figs. 145, 146.), which two genera seem wanting in the lower crag.

[4 Ill.u.s.trations: Fossils characteristic of the Red Crag.

Fig. 144. _Fusus contrarius._

Fig. 145. _Murex alveolatus._

Fig. 146. _Na.s.sa granulata._

Fig. 147. _Cypraea coccinelloides._

Fig. 144. half nat. size; the others nat. size.]

Among the bones and teeth of fishes are those of large sharks (_Carcharias_), and a gigantic skate of the extinct genus _Myliobates_, and many other forms, some common to our seas, and many foreign to them.

The distinctness of the fossils of the coralline crag arises in part from higher antiquity, and, in some degree, from a difference in the geographical conditions of the submarine bottom. The prolific growth of corals, echini, and a prodigious variety of testacea, implies a region of deeper and more tranquil water; whereas, the red crag may have formed afterwards on the same spot, when the water was shallower. In the mean time the climate may have become somewhat cooler, and some of the zoophytes which flourished in the first period may have disappeared, so that the fauna of the red crag acquired a character somewhat more nearly resembling that of our northern seas, as is implied by the large development of certain sections of the genera _Fusus_, _Buccinum_, _Purpura_, and _Trochus_, proper to higher lat.i.tudes, and which are wanting or feebly represented in the inferior crag.

Some of the corals of the lower crag of Suffolk belong to genera unknown in the living creation, and of a very peculiar structure; as, for example, that represented in the annexed fig. (148.), which is one of several species having a globular form. The great number and variety of these zoophytes probably indicate an equable climate, free from intense cold in winter. On the other hand, that the heat was never excessive is confirmed by the prevalence of northern forms among the testacea, such as the _Glycimeris_, _Cyprina_, and _Astarte_. Of the genus last mentioned (see fig. 149.) there are about fourteen species, many of them being rich in individuals; and there is an absence of genera peculiar to hot climates, such as _Conus_, _Oliva_, _Mitra_, _Fasciolaria_, _Cra.s.satella_, and others. The cowries (_Cypraea_, fig. 147.), also, are small, and belong to a section (_Trivia_) now inhabiting the colder regions. A large volute, called _Voluta Lamberti_ (fig. 150.), may seem an exception; but it differs in form from the volutes of the torrid zone, and may, like the living _Voluta Magellanica_, have been fitted for an extra-tropical climate.

[Ill.u.s.tration: Fig. 148. _Fascicularia aurantium_, Milne Edwards. Family, _Tubuliporidae_, of same author.

Coral of extinct genus, from the inferior or coralline crag, Suffolk.

_a._ exterior.

_b._ vertical section of interior.

_c._ portion of exterior magnified.

_d._ portion of interior magnified, showing that it is made up of long, thin, straight tubes, united in conical bundles.]

[Ill.u.s.tration: Fig. 149. _Astarte_ (_Cra.s.sina_, Lam.); species common to upper and lower crag.

_Astarte Omalii_, Lajonkaire; Syn. _A. bipart.i.ta_, Sow. Min. Con. T.

521. f. 3.; a very variable species most characteristic of the coralline crag, Suffolk.]

[Ill.u.s.tration: Fig. 150. _Voluta Lamberti_, young individ.]

The occurrence of a species of _Lingula_ at Sutton is worthy of remark, as these _Brachiopoda_ seem now confined to more equatorial lat.i.tudes, and the same may be said still more decidedly of a species of _Pyrula_, allied to _P. reticulata_. Whether, therefore, we may incline to the belief that the mean annual temperature was higher or lower than now, we may at least infer that the climate and geographical conditions were by no means the same at the period of the Suffolk crag as those now prevailing in the same region.

Of the echinoderms of the coralline crag about eleven species are known, but some of these are in too fragmentary a condition to admit of exact comparison. Of six which are the most perfect, Prof. E. Forbes has been able to identify three with recent species: one of which, of the genus _Echinus_, is British; a second, _Echinocyamus_, British and Mediterranean; and a third, _Echinus monilis_, a Mediterranean species, also found fossil in the faluns of Touraine.

One of the most interesting conclusions deduced from a careful comparison of the sh.e.l.ls of these British Older Pliocene strata and those now inhabiting our seas, has been pointed out by Prof. E. Forbes. It appears that, during the glacial period, a period intermediate, as we have seen, between that of the crag and our own times, many sh.e.l.ls, previously established in the temperate zone, retreated southwards to avoid an uncongenial climate. The Professor has given a list of fifty sh.e.l.ls which inhabited the British seas while the coralline and red crag were forming, and which are wanting in the Pleistocene or glacial deposits. They must, therefore, after their migration to the south, have made their way northwards again. In corroboration of these views, it is stated that all these fifty species occur fossil in the Newer Pliocene strata of Sicily, Southern Italy, and the Grecian Archipelago, where they may have enjoyed, during the era of floating icebergs, a climate resembling that now prevailing in higher European lat.i.tudes.[166-A]

In the red crag at Felixstow, in Suffolk, Professor Henslow has found the ear-bones of no less than four species of cetacea, which, according to Mr.

Owen, are the remains of true whales of the family _Balaenidae_. Mr. Wood is of opinion that these cetacea may be of the age of the red crag, or if not that they may be derived from the destruction of beds of coralline crag. I agree with him that the supposition of their having been washed out of the London clay, in which no _Balaenidae_ have yet been met with, is improbable.

Strata containing fossil sh.e.l.ls, like those of the Suffolk crag, above described, have been found at Antwerp, and on the banks of the Scheldt below that city. In 1840 I observed a small patch of them near Valognes, in Normandy; and there is also a deposit containing similar fossils at St.

George Bohon, and several places a few leagues to the S. of Carentan, in Normandy; but they have never been traced farther southwards.

_Subapennine strata._--The Apennines, it is well known, are composed chiefly of secondary 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, as we have seen (p. 105.), was the first Italian geologist who described this newer group in detail, giving it the name of the Subapennines; and he cla.s.sed all the tertiary strata of Italy, from Piedmont to Calabria, as parts of the same system. Certain mineral characters, he observed, were common to the whole; for the strata consist generally of light brown or blue marl, covered by yellow calcareous sand and gravel. There are also, he added, some species of fossil sh.e.l.ls which are found in these deposits throughout the whole of Italy.

We have now, however, satisfactory evidence that the Subapennine beds of Brocchi belong, at least, to three periods. To the Miocene we can refer a portion of the strata of Piedmont, those of the hill of the Superga, for example; to the Older Pliocene, part of the strata of northern Italy, of Tuscany, and of Rome; while the tufaceous formations of Naples, of Ischia, and the calcareous strata of Otranto, are referable to the Newer Pliocene, and in great part to the Post-Pliocene period.

That there is a considerable correspondence in the mineral composition of these different Italian groups is undeniable; but not that exact resemblance which should lead us to a.s.sume a precise ident.i.ty of age, unless the fossil remains agreed very closely. It is now indispensable that a new scrutiny should be made in each particular district, of the fossils derived from the upper and lower beds--especially such localities as Asti and Parma, where the formation attains a great thickness; and at Sienna, where the sh.e.l.ls of the inc.u.mbent yellow sand are generally believed to approach much more nearly, as a whole, to the recent fauna of the Mediterranean than those in the subjacent blue marl.