Part 39 (1/2)

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 Yorks.h.i.+re in the north-east, to Dorsets.h.i.+re 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 Yorks.h.i.+re 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 Dorsets.h.i.+re 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 Wilts.h.i.+re, 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.]