Part 30 (1/2)

The Crag, divided into three portions, is a local deposit of limited extent. It consists of variable beds of sand, gravel, and marl; sometimes it is a sh.e.l.ly ferruginous grit, as the Red Crag; at others a soft calcareous rock made up of sh.e.l.ls and bryozoa, as the Coralline Crag.

The _Coralline Crag_, of very limited extent in this country, ranges over about twenty miles between the rivers Stour and Alde, with a breadth of three or four. It consists of two divisions--an upper one, formed chiefly of the remains of Bryozoa, and a lower one of light-coloured sands, with a profusion of sh.e.l.ls. The upper division is about thirty-six feet thick at Sudbourne in Suffolk, where it consists of a series of beds almost entirely composed of comminuted sh.e.l.ls and remains of Bryozoa, forming a soft building-stone. The lower division is about forty-seven feet thick at Sutton; making the total thickness of the Coralline Crag about eighty-three feet.

Many of the Coralline Crag Mollusca belong to living species; they are supposed to indicate an equable climate free from intense cold--an inference rendered more probable by the prevalence of northern forms of sh.e.l.ls, such as _Glycimeris_, _Cyprina_, and _Astarte_. The late Professor Edward Forbes, to whom science is indebted for so many philosophical deductions, points out some remarkable inferences drawn from the fauna of the Pliocene seas.[95] It appears that in the glacial period, which we shall shortly have under consideration, many sh.e.l.ls, previously established in the temperate zone, retreated southwards, to avoid an uncongenial climate. The Professor gives a list of fifty which inhabited the British seas while the Coralline and Red Crag were forming, but which are all wanting in the glacial deposits;[96] from which he infers that they migrated at the approach of the glacial period, and returned again northwards, when the temperate climate was restored.[97]

[95] Edward Forbes in ”Memoirs of the Geological Survey of Great Britain,” vol. i., p. 336.

[96] For full information on these deposits the reader is referred to the ”Memoirs on the Structure of the Crag-beds of Norfolk and Suffolk,” by J. Prestwich, F.R.S., in the _Quart. Jour. Geol.

Soc._, vol. xxvii., pp. 115, 325, and 452 (1871). Also to the many Papers by the Messrs. Searles Wood published in the _Quar.

Jour. Geol. Soc._, the _Ann. Nat. Hist._, the _Phil. Mag._, &c.

[97] Lyell's ”Elements of Geology,” p. 203.

In the Upper or Mammaliferous (or Norwich) Crag, of which there is a good exposure in a pit near the asylum at Thorpe, bones of Mammalia are found with existing species of sh.e.l.ls. The greater number of the Mammalian remains have been supposed, until lately, to be extraneous fossils; but they are now considered by Mr. Prestwich as truly contemporaneous. The peculiar mixture of southern forms of life with others of a more northern type lead to the inference that, at this early period, a lowering of temperature began gradually to set in from the period of the Coralline Crag to that of the Forest Bed, which marks the commencement of the Glacial Period.

The distinction between the Mammaliferous Crag of Norwich and the Red Crag of Suffolk is purely palaeontological, no case of superposition having yet been discovered, and they are now generally considered as contemporaneous. Two Proboscidians abundant during the Crag period were the _Mastodon Arvernensis_ and the _Elephas meridionalis_. In the Red Crag the Mastodon is stated by the Rev. John Gunn to be more abundant than the Elephant, while in the Norwich beds their proportions are nearly equal.

At or near the base of the Red Crag there is a remarkable acc.u.mulation, varying in thickness from a few inches to two feet, of bones, teeth, and phosphatic nodules (called coprolites), which are worked for making superphosphate of lime for agricultural manure.

The foreign equivalents of the older Pliocene are found in the _sub-Apennine strata_. These rocks are sufficiently remarkable in the county of Suffolk, where they consist of a series of marine beds of quartzose sand, coloured red by ferruginous matter.

At the foot of the Apennine chain, which forms the backbone, as it were, of Italy, throwing out many spurs, the formations on either side, and on both sides of the Adriatic, are Tertiary strata; they form in many cases, low hills lying between the Apennines of Secondary formation and the sea, the strata generally being a light-brown or bluish marl covered with yellow calcareous sand and gravel, with some fossil sh.e.l.ls, which, according to Brocchi, are found all over Italy. But this wide range includes some older Tertiary formations, as in the strata of the Superga near Turin, which are Miocene.

The _Antwerp_ Crag, which is of the same age with the Red and Coralline Crag of Suffolk, forms great acc.u.mulations upon divers points of Europe: at Antwerp in Belgium, at Carentan and Perpignan, and, we believe, in the basin of the Rhone, in France. The thickest deposits of this rock consist of clay and sand, alternating with marl and arenaceous limestone. These const.i.tute the sub-Apennine hills, alluded to above as extending on both slopes of the Apennines. This deposit occupies the Upper Val d'Arno, above Florence. Its presence is recognised over a great part of Australia. Finally, the seven hills of Rome are composed, in part, of marine Tertiary rocks belonging to the Pliocene period.

In PLATE XXV. an ideal landscape of the Pliocene period is given under European lat.i.tudes. In the background of the picture, a mountain, recently thrown up, reminds us that the period was one of frequent convulsions, in which the land was disturbed and upheaved, and mountains and mountain-ranges made their appearance. The vegetation is nearly identical with the present. We see a.s.sembled in the foreground the more important animals of the period--the fossil species, as well as those which have survived to the present time.

At the close of the Pliocene period, and in consequence of the deposits left by the seas of the Tertiary epoch, the continent of Europe was nearly what it is now; few permanent changes have occurred since to disturb its general outline. Although the point does not admit of actual proof, there is strong presumptive evidence that in this period, or in that immediately subsequent to it, the entire European area, with some trifling exceptions, including the Alps and Apennines, emerged from the deep. In Sicily, Newer Pliocene rocks, covering nearly half the surface of the island, have been raised from 2,000 to 3,000 feet above the level of the sea. Fossil sh.e.l.ls have been observed at the height of 8,000 feet in the Pyrenees; and, as if to fix the date of upheaval, there are great ma.s.ses of granite which have penetrated the Lias and the Chalk. Fossil sh.e.l.ls of the period are also found at a height of 10,000 feet in the Alps, at 13,000 feet in the Andes, and at 18,000 feet in the Himalayas.

[Ill.u.s.tration: XXV.--Ideal Landscape of the Pliocene Period.]

In the mountainous regions of the Alps it is always difficult to determine the age of beds, in consequence of the disturbed state of the strata; for instance, the lofty chain of the Swiss Jura consists of many parallel ridges, with intervening longitudinal valleys; the ridges formed of contorted fossiliferous strata, which are extensive in proportion to the number and thickness of the formations which have been exposed on upheaval. The proofs which these regions offer of comparatively recent elevation are numerous. In the central Alps, Cretaceous, Oolitic, Lia.s.sic, and Eocene strata are found at the loftiest summits, pa.s.sing insensibly into metamorphic rocks of granular limestone, and into talcose and mica-schists. In the eastern parts of the chain the older fossiliferous rocks are recognised in similar positions, presenting signs of intense Plutonic action. Oolitic and Cretaceous strata have been raised 12,000 feet, Eocene 10,000, and Miocene 4,000 and 5,000 feet above the level of the sea. Equally striking proofs of recent elevation exist in the Apennines; the celebrated Carrara marble, once supposed--from its crystalline texture and the absence of fossils, and from its resting--1. on talcose schists, 2. on quartz and gneiss--to be very ancient, now turns out to be an altered limestone of the Oolitic series, and the underlying crystalline rocks to be metamorphosed Secondary sandstones and shales. Had all these rocks undergone complete metamorphism, another page in the earth's history would have been obscured. As it is, the proofs of what we state are found in the gradual approach of the rocks to their unaltered condition as the distance from the intrusive rock increases. This intrusive rock, however, does not always reach the surface, but it exists below at no great depth, and is observed piercing through the talcose gneiss, and pa.s.sing up into Secondary strata.

At the close of this epoch, therefore, there is every probability that Europe and Asia had pretty nearly attained their present general configuration.

QUATERNARY EPOCH.

The Quaternary epoch of the history of our globe commences at the close of the Tertiary epoch, and brings the narrative of its revolutions down to our own times.

The tranquillity of the globe was only disturbed during this era by certain cataclysms whose sphere was limited and local, and by an interval of cold of very extended duration; the _deluges_ and the _glacial_ period--these are the two most remarkable peculiarities which distinguished this epoch. But the fact which predominates in the Quaternary epoch, and distinguishes it from all other phases of the earth's history is the appearance of man, the culminating and supreme work of the Creator of the universe.

In this last phase of the history of the earth geology recognises three chronological divisions:--

1. The European Deluges.

2. The Glacial Period.

3. The creation of man and subsequent Asiatic Deluge.