A Manual of Elementary Geology Part 39

occurs in England and many other parts of Europe. This group has been so named, because, in the countries where it was first examined, the limestones belonging to it had an oolitic structure (see p. 12.). These rocks occupy in England a zone which is nearly 30 miles in average breadth, and extends across the island, from Yorkshire in the north-east, to Dorsetshire in the south-west. Their mineral characters are not uniform throughout this region; but the following are the names of the princ.i.p.al subdivisions observed in the central and south-eastern parts of England:--

OOLITE.

Upper { _a._ Portland stone and sand.

{ _b._ Kimmeridge clay.

Middle { _c._ Coral rag.

{ _d._ Oxford clay.

Lower { _e._ Cornbrash and Forest marble.

{ _f._ Great Oolite and Stonesfield slate.

{ _g._ Fuller's earth.

{ _h._ Inferior Oolite.

The Lias then succeeds to the Inferior Oolite.

The Upper oolitic system of the above table has usually the Kimmeridge clay for its base; the Middle oolitic system, the Oxford clay. The Lower system reposes on the Lias, an argillo-calcareous formation, which some include in the Lower Oolite, but which will be treated of separately in the next chapter. Many of these subdivisions are distinguished by peculiar organic remains; and though varying in thickness, may be traced in certain directions for great distances, especially if we compare the part of England to which the above-mentioned type refers with the north-east of France, and the Jura mountains adjoining. In that country, distant above 400 geographical miles, the a.n.a.logy to the English type, notwithstanding the thinness, or occasional absence of the clays, is more perfect than in Yorkshire or Normandy.

_Physical geography._--The alternation, on a grand scale, of distinct formations of clay and limestone, has caused the oolitic and lia.s.sic series to give rise to some marked features in the physical outline of parts of England and France. Wide valleys can usually be traced throughout the long bounds of country where the argillaceous strata crop out; and between these valleys the limestones are observed, composing ranges of hills, or more elevated grounds. These ranges terminate abruptly on the side on which the several clays rise up from beneath the calcareous strata.

[Ill.u.s.tration: Fig. 266. Cross section.]

The annexed diagram will give the reader an idea of the configuration of the surface now alluded to, such as may be seen in pa.s.sing from London to Cheltenham, or in other parallel lines, from east to west, in the southern part of England. It has been necessary, however, in this drawing, greatly to exaggerate the inclination of the beds, and the height of the several formations, as compared to their horizontal extent. It will be remarked, that the lines of cliff, or escarpment, face towards the west in the great calcareous eminences formed by the Chalk and the Upper, Middle, and Lower Oolites; and at the base of which we have respectively the Gault, Kimmeridge clay, Oxford clay, and Lias. This last forms, generally, a broad vale at the foot of the escarpment of inferior oolite, but where it acquires considerable thickness, and contains solid beds of marlstone, it occupies the lower part of the escarpment.

The external outline of the country which the geologist observes in travelling eastward from Paris to Metz is precisely a.n.a.logous, and is caused by a similar succession of rocks intervening between the tertiary strata and the Lias; with this difference, however, that the escarpments of Chalk, Upper, Middle, and Lower Oolites, face towards the east instead of the west.

The Chalk crops out from beneath the tertiary sands and clays of the Paris basin, near Epernay, and the Gault from beneath the Chalk and Upper Greensand at Clermont-en-Argonne; and pa.s.sing from this place by Verdun and Etain to Metz, we find two limestone ranges, with intervening vales of clay, precisely resembling those of southern and central England, until we reach the great plain of Lias at the base of the Inferior Oolite at Metz.

It is evident, therefore, that the denuding causes have acted similarly over an area several hundred miles in diameter, sweeping away the softer clays more extensively than the limestones, and undermining these last so as to cause them to form steep cliffs wherever the harder calcareous rock was based upon a more yielding and destructible clay. This denudation probably occurred while the land was slowly rising out of the sea.[259-A]

_Upper Oolite._

The Portland stone has already been mentioned as forming in Dorsetshire the foundation on which the freshwater limestone of the Lower Purbeck reposes (see p. 232.). It supplies the well-known building stone of which St. Paul's and so many of the princ.i.p.al edifices of London are constructed. This upper member, characterized by peculiar marine fossils, rests on a dense bed of sand, called the Portland sand, below which is the Kimmeridge clay. In England these Upper Oolite formations are almost wholly confined to the southern counties. Corals are rare in them, although one species is found plentifully at Tisbury, in Wiltshire, in the Portland sand converted into flint and chert, the original calcareous matter being replaced by silex (fig. 267.).

[Ill.u.s.tration: Fig. 267. _Columnaria oblonga_, Blainv.

As seen on a polished slab of chert from the sand of the Upper Oolite, Tisbury.]

Among the characteristic fossils of the Upper Oolite, may be mentioned the _Ostrea deltoidea_ (fig. 269.), found in the Kimmeridge clay throughout England and the north of France, and also in Scotland, near Brora. The _Gryphaea virgula_ (fig. 268.), also met with in the same clay near Oxford, is so abundant in the Upper Oolite of parts of France as to have caused the deposit to be termed "marnes a gryphees virgules." Near Clermont, in Argonne, a few leagues from St. Menehould, where these indurated marls crop out from beneath the gault, I have seen them, on decomposing, leave the surface of every ploughed field literally strewed over with this fossil oyster.

[2 Ill.u.s.trations: Upper Oolite: Kimmeridge clay. 1/4 nat. size.

Fig. 268. _Gryphaea virgula._

Fig. 269. _Ostrea deltoidea._]

[Ill.u.s.tration: Fig. 270. _Trigonia gibbosa._ 1/2 nat. size. _a._ the hinge.

Portland Oolite, Tisbury.]

The Kimmeridge clay consists, in great part, of a bituminous shale, sometimes forming an impure coal several hundred feet in thickness. In some places in Wiltshire it much resembles peat; and the bituminous matter may have been, in part at least, derived from the decomposition of vegetables. But as impressions of plants are rare in these shales, which contain ammonites, oysters, and other marine sh.e.l.ls, the bitumen may perhaps be of animal origin.

The celebrated lithographic stone of Solenhofen, in Bavaria, belongs to one of the upper divisions of the oolite, and affords a remarkable example of the variety of fossils which may be preserved under favourable circ.u.mstances, and what delicate impressions of the tender parts of certain animals and plants may be retained where the sediment is of extreme fineness. Although the number of testacea in this slate is small, and the plants few, and those all marine, Count Munster had determined no less than 237 species of fossils when I saw his collection in 1833; and among them no less than seven _species_ of flying lizards, or pterodactyls, six saurians, three tortoises, sixty species of fish, forty-six of crustacea, and twenty-six of insects. These insects, among which is a libellula, or dragon-fly, must have been blown out to sea, probably from the same land to which the flying lizards, and other contemporaneous reptiles, resorted.

_Middle Oolite._

_Coral Rag._--One of the limestones of the Middle Oolite has been called the "Coral Rag," because it consists, in part, of continuous beds of petrified corals, for the most part retaining the position in which they grew at the bottom of the sea. They belong chiefly to the genera _Caryophyllia_ (fig. 271.), _Agaricia_, and _Astrea_, and sometimes form ma.s.ses of coral 15 feet thick. In the annexed figure of an _Astrea_, from this formation, it will be seen that the cup-shaped cavities are deepest on the right-hand side, and that they grow more and more shallow, till those on the left side are nearly filled up. The last-named stars are supposed to be Polyparia of advanced age. These coralline strata extend through the calcareous hills of the N.W. of Berkshire, and north of Wilts, and again recur in Yorkshire, near Scarborough.

[Ill.u.s.tration: Fig. 271. _Caryophyllia annularis_, Parkin. Coral rag, Steeple Ashton.]

[Ill.u.s.tration: Fig 272. _Astrea._ Coral rag.]

One of the limestones of the Jura, referred to the age of the English coral rag, has been called "Nerinaean limestone" (Calcaire a Nerinees) by M. Thirria; _Nerinaea_ being an extinct genus of univalve sh.e.l.ls, much resembling the _Cerithium_ in external form. The annexed section (fig.

273.) shows the curious form of the hollow part of each whorl, and also the perforation which pa.s.ses up the middle of the columella. _N.

Goodhallii_ (fig. 274.) is another English species of the same genus, from a formation which seems to form a pa.s.sage from the Kimmeridge clay to the coral rag.[261-A]

[Ill.u.s.tration: Fig. 273. _Nerinaea hieroglyphica._ Coral rag.]

[Ill.u.s.tration: Fig. 274. _Nerinaea Goodhallii_, Fitton. Coral rag, Weymouth. 1/4 nat. size.]

A division of the oolite in the Alps, regarded by most geologists as coeval with the English coral rag, has been often named "Calcaire a Dicerates," or "Diceras limestone," from its containing abundantly a bivalve sh.e.l.l (see fig. 275.) of a genus allied to the _Chama_.

[Ill.u.s.tration: Fig. 275. Cast of _Diceras arietina_. Coral rag, France.]

[Ill.u.s.tration: Fig. 276. _Cidaris coronata._ Coral rag.]

_Oxford Clay._--The coralline limestone, or "coral rag," above described, and the accompanying sandy beds, called "calcareous grits" of the Middle Oolite, rests on a thick bed of clay, called the Oxford clay, sometimes not less than 500 feet thick. In this there are no corals, but great abundance of cephalopoda of the genera Ammonite and Belemnite. (See fig. 277.) In some of the clay of very fine texture ammonites are very perfect, although somewhat compressed, and are seen to be furnished on each side of the aperture with a single horn-like projection (see fig. 278.). These were discovered in the cuttings of the Great Western Railway, near Chippenham, in 1841, and have been described by Mr. Pratt.[262-A]

[Ill.u.s.tration: Fig. 277. _Belemnites hastatus._ Oxford Clay.]

[Ill.u.s.tration: Fig. 278. _Ammonites Jason_, Reinecke. Syn. _A. Elizabethae_, Pratt. Oxford clay, Christian Malford, Wiltshire.]

[Ill.u.s.tration: Fig. 279. _Belemnites Puzosia.n.u.s_, D'Orb. Oxford Clay, Christian Malford.

_a, a._ projecting processes of the sh.e.l.l or phragmocone.

_b, c._ broken exterior of a conical sh.e.l.l called the phragmocone, which is chambered within, or composed of a series of shallow concave cells pierced by a siphuncle.

_c, d._ The guard or osselet, which is commonly called the belemnite.]

Similar elongated processes have been also observed to extend from the sh.e.l.ls of some belemnites discovered by Dr. Mantell in the same clay (see fig. 279.), who, by the aid of this and other specimens, has been able to throw much light on the structure of this singular extinct form of cuttle-fish.[263-A]

_Lower Oolite._

The upper division of this series, which is more extensive than the preceding or Middle Oolite, is called in England the Cornbrash. It consists of clays and calcareous sandstones, which pa.s.s downwards into the Forest marble, an argillaceous limestone, abounding in marine fossils. In some places, as at Bradford, this limestone is replaced by a ma.s.s of clay. The sandstones of the Forest Marble of Wiltshire are often ripple-marked and filled with fragments of broken sh.e.l.ls and pieces of drift-wood, having evidently been formed on a coast. Rippled slabs of fissile oolite are used for roofing, and have been traced over a broad band of country from Bradford, in Wilts, to Tetbury, in Gloucestershire.

These calcareous tile-stones are separated from each other by thin seams of clay, which have been deposited upon them, and have taken their form, preserving the undulating ridges and furrows of the sand in such complete integrity, that the impressions of small footsteps, apparently of crabs, which walked over the soft wet sands, are still visible. In the same stone the claws of crabs, fragments of echini, and other signs of a neighbouring beach are observed.[263-B]

_Great Oolite._--Although the name of coral-rag has been appropriated, as we have seen, to a member of the Upper Oolite before described, some portions of the Lower Oolite are equally int.i.tled in many places to be called coralline limestones. Thus the Great Oolite near Bath contains various corals, among which the _Eunomia radiata_ (fig. 280.) is very conspicuous, single individuals forming ma.s.ses several feet in diameter; and having probably required, like the large existing brain-coral (_Meandrina_) of the tropics, many centuries before their growth was completed.