Part 25 (1/2)

[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._