The rocks which now represent the Middle Oolitic Period are usually divided into the _Oxford Clay_, the lower member of which is an arenaceous limestone, known as the _Kellaways Rock_, which in Wiltshire and other parts of the south-west of England attains a thickness of eight or ten feet, with the impressions of numerous Ammonites, and other sh.e.l.ls. In Yorkshire, around Scarborough, it reaches the thickness of thirty feet; and forms well-developed beds of bluish-black marl in the department of Calvados, in France. It is the base of this clay which forms the soil (_Argile de Dives_) of the valley of the Auge, renowned for its rich pasturages and magnificent cattle. The same beds form the base of the oddly-shaped but fine rocks of La Manche, which are popularly known as the _Vaches Noires_ (or black cows)--a locality celebrated, also, for its fine Ammonites transformed into pyrites.

The _Oxford Clay_ const.i.tutes the base of the hills in the neighbourhood of Oxford, forming a bed of clay sometimes more than 600 feet thick. It is found well-developed in France, at Trouville, in the department of the Calvados; and at Neuvisy, in the department of the Ardennes, where it attains a thickness of about 300 feet. It is a bluish, sometimes whitish limestone (often argillaceous), and bluish marl. The _Gryphaea dilatata_ is the most common fossil in the Oxford Clay. The _Coral Rag_ is so called from the fact that the limestone of which it is chiefly composed consists, in part, of an aggregation of considerable ma.s.ses of petrified Corals; not unlike those now existing in the Pacific Ocean, supposing them to be covered up for ages and fossilised. This coral stratum extends through the hills of Berkshire and North Wilts, and it occurs again near Scarborough. In the counties of Dorset, Bedford, Buckingham, and Cambridge, and some other parts of England, the limestone of the Coral Rag disappears and is replaced by clay--in which case the Oxford Clay is overlaid directly by the Kimeridge Clay. In France it is found in the departments of the Meuse, of the Yonne, of the Ain, of the Charente Inferieure. In the Alps the _Diceras limestone_ is regarded, by most geologists, as coeval with the English Coral Rag.

UPPER OOLITE.

[Ill.u.s.tration: Fig. 122.--Bird of Solenhofen (Archaeopteryx).]

Some marsupial Mammals have left their remains in the Upper Oolite as in the Lower. They belong to the genus _Sphalacotherium_. Besides the Plesiosauri and Teleosauri, there still lived in the maritime regions a Crocodile, the _Macrorhynchus_; and the monstrous _Pcilopleuron_, with sharp cutting teeth, one of the most formidable animals of this epoch; the _Hylaeosaurus_, _Cetiosaurus_, _Stenosaurus_, and _Streptospondylus_, and among the Turtles, the _Emys_ and _Platemys_. As in the Lower Oolite, so also in the Upper, Insects similar to those by which we are surrounded, pursued their flight in the meadows and hovered over the surface of the water. Of these, however, too little is known for us to give any very precise indication on the subject of their special organisation.

The most remarkable fact relating to this period is the appearance of the first bird. Hitherto the Mammals, and of these only imperfectly-organised species, namely, the Marsupials, have alone appeared. It is interesting to witness birds appearing immediately after. In the quarries of lithographic stone at Solenhofen, the remains of a bird, with feet and feathers, have been found, but without the head. These curious remains are represented in Fig. 122, in the position in which they were discovered. The bird is usually designated the Bird of Solenhofen.

[Ill.u.s.tration: Fig. 123.

Sh.e.l.l of Physa fontinalis.]

The Oolitic seas of this series contained Fishes belonging to the genera _Asteracanthus_, _Strephodes_, _Lepidotus_, and _Microdon_. The Cephalopodous Mollusca were not numerous, the predominating genera belonging to the Lamellibranchs and to the Gasteropods, which lived on the sh.o.r.e. The reef-making Madrepores or Corals were more numerous. A few Zoophytes in the fossil state testify to the existence of these extraordinary animals. The fossils characteristic of the fauna of the period include _Ammonites decipiens_ and _A. giganteus_, _Natica elegans_ and _hemispherica_, _Ostrea deltoidea_ and _O. virgula_, _Trigonia gibbosa_, _Pholadomya multicostata_ and _P. acuticostata_, _Terebratula subsella_, and _Hemicidaris Purbeckensis_. Some _Fishes_, _Turtles_, _Paludina_, _Physa_ (Fig. 123), _Unio_, _Planorbis_ (Fig.

201), and the little crustacean bivalves, the Cypris, const.i.tuted the fresh-water fauna of the period.

The terrestrial flora of the period consisted of Ferns, Cycadeaceae, and Conifers; in the ponds and swamps some Zosterae. The _Zosterae_ are monocotyledonous plants of the family of the Nadaceae, which grow in the sandy mud of maritime regions, forming there, with their long, narrow, and ribbon-like leaves, vast prairies of the most beautiful green. At low tides these ma.s.ses of verdure appear somewhat exposed. They would form a retreat for a great number of marine animals, and afford nourishment to others.

[Ill.u.s.tration: XX.--Ideal Landscape of the Upper Oolitic Period.]

On the opposite page an ideal landscape of the period (PLATE XX.) represents some of the features of the Upper Oolite, especially the vegetation of the Jura.s.sic period. The _Sphenophyllum_, among the Tree-ferns, is predominant in this vegetation; some _Pandanas_, a few _Zamites_, and many _Conifers_, but we perceive no Palms. A coral islet rises out of the sea, having somewhat of the form of the _atolls_ of Oceania, indicating the importance these formations a.s.sumed in the Jura.s.sic period. The animals represented are the _Crocodileimus_ of Jourdan, the _Ramphorynchus_, with the imprints which characterise its footsteps, and some of the invertebrated animals of the period, as the _Asteria_, _Comatula_, _Hemicidaris_, _Pteroceras_. Aloft in the air floats the bird of Solenhofen, the _Archaeopteryx_, which has been re-constructed from the skeleton, with the exception of the head, which remains undiscovered.

The rocks which represent the Upper Oolite are usually divided into two series: 1. The _Purbeck Beds_; 2. The _Portland Stone and Sand_; and 3.

The _Kimeridge Clay_.

The _Kimeridge Clay_, which in many respects bears a remarkable resemblance to the Oxford Clay, is composed of blue or yellowish argillaceous beds, which occur in the state of clay and shale (containing locally beds of bituminous schist, sometimes forming a sort of earthy impure coal), and several hundred feet in thickness. These beds are well developed at Kimeridge, in Dorsetshire, whence the clay takes its name. In some parts of Wiltshire the beds of bituminous matter have a shaly appearance, but there is an absence of the impressions of plants which usually accompany the bitumen, derived from the decomposition of plants. These rocks, with their characteristic fossils, _Cardium striatulum_ and _Ostrea deltoidea_, are found throughout England: in France, at Tonnerre, Dept. Yonne; at Havre; at Honfleur; at Mauvage; in the department of the Meuse it is so rich in sh.e.l.ls of _Ostrea deltoidea_ and _O. virgula_, that, "near Clermont in Argonne, a few leagues from St. Menehould," says Lyell,[70] "where these indurated marls crop out from beneath the Gault, I have seen them (_Gryphaea virgula_) on decomposing leave the surface of every ploughed field literally strewed over with this fossil oyster."

[70] "Elements of Geology," p. 393.

The second section of this series consists of the oolitic limestone of Portland, which is quarried in the Isle of Portland and in the cliffs of the Isle of Purbeck in Dorsetshire, and also at Chilmark in the Vale of Wardour, in Wiltshire. In France, the Portland beds are found near Boulogne, at Cirey-le-Chateau, Auxerre, and Gray (Haute Saone).

The Isle, or rather peninsula of Portland,[71] off the Dorsetshire coast, rises considerably above the sea-level, presenting on the side of the port a bold line of cliffs, connected with the mainland by the Chesil bank,[72] an extraordinary formation, consisting of a beach of shingle and pebbles loosely piled on the blue Kimeridge clay, and stretching ten miles westward along the coast. The quarries are chiefly situated in the northerly part of the island. The story told of this remarkable island is an epitome of the revolutions the surface of the earth has undergone. The slaty Purbeck beds which overlie the Portland stone are of a dark-yellowish colour; they are burnt in the neighbourhood for lime. The next bed is of a whiter and more lively colour. It is the stone of which the portico of St. Paul's and many of the houses of London, built in Queen Anne's time, were constructed. The building-stone contains fossils exclusively marine. Upon this stratum rests a bed of limestone formed in lacustrine waters. Finally, upon this bed rests another deposit of a substance which consists of very well-preserved vegetable earth or _humus_, quite a.n.a.logous to our vegetable soil, of the thickness of from fifteen to eighteen inches, and of a blackish colour; it contains a strong proportion of carbonaceous earth; it abounds in the silicified remains of Conifers and other plants, a.n.a.logous to the _Zamia_ and _Cycas_--this soil is known as the "dirt-bed." The trunks of great numbers of silicified trees and tropical plants are found here erect, their roots fixed in the soil, and of species differing from any of our forest trees. "The ruins of a forest upon the ruins of a sea," says Esquiros, "the trunks of these trees were petrified while still growing. The region now occupied by the narrow channel and its environs had been at first a sea, in whose bed the Oolitic deposits which now form the Portland stone acc.u.mulated: the bed of the sea gradually rose and emerged from the waves. Upon the land thus rescued from the deep, plants began to grow; they now const.i.tute with their ruins the soil of the dirt-bed. This soil, with its forest of trees, was afterwards plunged again into the waters--not the bitter waters of the ocean, but in the fresh waters of a lake formed at the mouth of some great river."

[71] For details respecting these strata the reader may consult, with advantage, the useful handbook to the geology of Weymouth and Portland, by Robert Damon.

[72] See Bristow and Whitaker "On the Chesil Bank," _Geol. Mag._, vol.

vi., p. 433.

Time pa.s.sed on, however; a calcareous sediment brought from the interior by the waters, formed a layer of mud over the dirt-bed; finally, the whole region was covered by a succession of calcareous deposits, until the day when the Isle of Portland was again revealed to light. "From the facts observed," says Lyell, "we may infer:--1. That those beds of the Upper Oolite, called the Portland, which are full of marine sh.e.l.ls, were overspread with fluviatile mud, which became dry land, and covered with a forest, throughout a portion of s.p.a.ce now occupied by the south of England, the climate being such as to admit of the growth of the _Zamia_ and _Cycas_. 2. This land at length sank down and was submerged with its forest beneath a body of fresh water from which sediment was thrown down enveloping fluviatile sh.e.l.ls. 3. The regular and uniform preservation of this thin bed of black earth over a distance of many miles, shows that the change from dry land to the state of a fresh-water lake, or estuary, was not accompanied by any violent denudation or rush of water, since the loose black earth, together with the trees which lay prostrate on its surface, must inevitably have been swept away had any such violent catastrophe taken place."[73]

[73] "Elements of Geology," p. 389.

[Ill.u.s.tration: Fig. 124.--Geological humus. _a_, Fresh-water calcareous slate (Purbeck); _b_, Dirt-bed, with roots and stems of trees; _c_, Fresh-water beds; _d_, Portland Stone.]

The soil known as the _dirt-bed_ is nearly horizontal in the Isle of Portland; but we discover it again not far from there in the sea-cliffs of the Isle of Purbeck, having an inclination of 45, where the trunks continue perfectly parallel among themselves, affording a fine example of a change in the position of beds originally horizontal. Fig. 124 represents this species of geological _humus_. "Each _dirt-bed_" says Sir Charles Lyell, "may, no doubt, be the memorial of many thousand years or centuries, because we find that two or three feet of vegetable soil is the only monument which many a tropical forest has left of its existence ever since the ground on which it now stands was first covered with its shade."[74]

[74] Ibid, p. 391.

This bed of vegetable soil is, then, near the summit of that long and complicated series of beds which const.i.tute the Jura.s.sic period; these ruins, still vegetable, remind us forcibly of the coal-beds, for they are nothing else than a less advanced state of that kind of vegetable fossilisation which was perfected on such an immense scale, and during an infinite length of time in the coal period.

The Purbeck beds, which are sometimes subdivided into Lower, Middle, and Upper, are mostly fresh-water formations, intimately connected with the Upper Portland beds. But there they begin and end, being scarcely recognisable except in Dorsetshire, in the sea-cliffs of which they were first studied. They are finely exposed in Durdlestone Bay, near Swanage, and at Lulworth Cove, on the same coast. The _lower beds_ consist of a purely fresh-water marl, eighty feet thick, containing sh.e.l.ls of _Cypris_, _Limnaea_, and some _Serpulae_ in a bed of marl of brackish-water origin, and some _Cypris_-bearing shales, strangely broken up at the west end of the Isle of Purbeck.

The _Middle series_ consists of twelve feet of marine strata known as the "cinder-beds," formed of a vast acc.u.mulation of _Ostrea distorta_, resting on fresh-water strata full of _Cypris fasciculata_, _Planorbis_, and _Limnaea_, by which this strata has been identified as far inland as the vale of Wardour in Wiltshire. Above the cinder-beds are shales and limestones, partly of fresh-water and partly of brackish-water origin, in which are Fishes, many species of Lepidotus, and the crocodilian reptile, _Macrorhynchus_. On this rests a purely marine deposit, with _Pecten_, _Avicula_, &c. Above, again, are brackish beds with _Cyrena_, overlying which is thirty feet of fresh-water limestone, with _Fishes_, _Turtles_, and _Cyprides_.

The _upper beds_ are purely fresh-water strata, about fifty feet thick, containing _Paludina_, _Physa_, _Limnaea_, all very abundant. In these beds the Purbeck marble, formerly much used in the ornamental architecture of the old English cathedrals, was formerly quarried. (See Note, page 274.)

A few words may be added, in explanation of the term _oolite_, applied to this sub-period of the Jura.s.sic formation. In a great number of rocks of this series the elements are neither crystalline nor amorphous--they are, as we have already said, oolitic; that is to say, the ma.s.s has the form of the roe of certain fishes. The question naturally enough arises, Whence this singular oolitic structure a.s.sumed by the components of certain rocks? It is a.s.serted that the grinding action of the sea acting upon the precipitated limestone produces rounded forms a.n.a.logous to grains of sand. This hypothesis may be well founded in some cases. The marine sediments which are deposited in some of the warm bays of Teneriffe are found to take the spheroidal granulated form of the oolite. But these local facts cannot be made to apply to the whole extent of the oolitic formations. We must, therefore, look further for an explanation of the phenomena.

It is admitted that if the cascades of Tivoli, for example, can give birth to the oolitic grains, the same thing happens in the quietest basins, that in stalact.i.te-caverns oolitic grains develop themselves, which afterwards, becoming cemented together from the continued, but very slow, affluence of the calcareous waters, give rise to certain kinds of oolitic rocks.

On the other hand, it is known that nodules, more or less large, develop themselves in marls in consequence of the concentration of the calcareous elements, without the possibility of any wearing action of water. Now, as there exists every gradation of size between the smallest oolitic grains and the largest concretions, it is reasonable to suppose that the oolites are equally the product of concentration.

Finally, from research to research, it is found that perfectly const.i.tuted oolites--that is to say, concentric layers, as in the Jura.s.sic limestone--develop themselves in vegetable earth in places where the effects of water in motion is not more admissible than in the preceding instances.

Thus we arrive at the conclusion, that if Nature sometimes forms crystals with perfect terminations in magmas in the course of solidification, she gives rise also to spheroidal forms surrounding various centres, which sometimes originate spontaneously, and in other cases are acc.u.mulated round the debris of fossils, or even mere grains of sand. Nevertheless, all mineral substances are not alike calculated to produce oolitic rocks; putting aside some particular cases, this property is confined to limestone and oxide of iron.

With regard to the distribution of the Jura.s.sic formation on the terrestrial globe, it may be stated that the Cotteswold Hills in England, and in France the Jura mountains, are almost entirely composed of these rocks, the several series of beds being all represented in them--this circ.u.mstance, in fact, induced Von Humboldt to name the formation after this latter range. The Upper Lias also exists in the Pyrenees and in the Alps; in Spain; in many parts of Northern Italy; in Russia, especially in the government of Moscow, and in the Crimea; but it is in Germany where it occupies the most important place. A thin bed of oolitic limestone presents, at Solenhofen in Bavaria, a geological repository of great celebrity, containing fossil Plants, Fishes, Insects, Crustaceans, with some Pterodactyles, admirably preserved; it yielded also some of the earliest of the feathered race. The fine quarries of lithographic stone at Pappenheim, so celebrated all over Europe, belong to the Jura.s.sic formation.

It has recently been announced that these rocks have been found in India; they contribute largely to the formation of the main ma.s.s of the Himalayas, and to the chain of the Andes in South America; finally, from recent investigations, they seem to be present in New Zealand.

In England the Lias const.i.tutes a well-defined belt about thirty miles broad, extending from Dorsetshire, in the south, to Yorkshire, in the north, formed of alternate beds of clay, shales, and limestone (with layers of jet), on the coast near Whitby. It is rich, as we have seen, in ancient life, and that in the strongest forms imaginable. From the unequal hardness of the rocks it comprises, it stands out boldly in some of the minor ranges of hills, adding greatly to the picturesque beauty of the scenery in the centre of the country. In Scotland the formation occupies a very limited s.p.a.ce.

A map of the country at the close of the Jura.s.sic period would probably show double the extent of dry land in the British Islands, compared with what it displayed as an island in the primordial ocean; but Devon and Cornwall had long risen from the sea, and it is probable that the Jura.s.sic beds of Dorsetshire and France were connected by a tongue of land running from Cherbourg to the Lia.s.sic beds of Dorsetshire, and that Boulogne, still an island, was similarly connected with the Weald.

[Ill.u.s.tration: Fig. 125.--Crioceras Duvallii, Sowerby. A non-involuted Ammonite. (Neocomian.)]

NOTE.--Sections of the Purbeck strata of Dorsetshire have been constructed by Mr. Bristow, from actual measurement, in the several localities in the Isle of Purbeck, where they are most clearly and instructively displayed.

These sections, published by the Geological Survey, show in detail the beds in their regular and natural order of succession, with the thickness, mineral character, and contents, as well as the fossils, of each separate bed.

THE CRETACEOUS PERIOD.

The name _Cretaceous_ (from _creta_, chalk) is given to this epoch in the history of our globe because the rocks deposited by the sea, towards its close, are almost entirely composed of chalk (carbonate of lime).

Carbonate of lime, however, does not now appear for the first time as a part of the earth's crust; we have already seen limestone occurring, among the terrestrial materials, from the Silurian period; the Jura.s.sic formation is largely composed of carbonate of lime in many of its beds, which are enormous in number as well as extent; it appears, therefore, that in the period called _Cretaceous_ by geologists, carbonate of lime was no new substance in the const.i.tution of the globe. If geologists have been led to give this name to the period, it is because it accords better than any other with the characteristics of the period; with the vast acc.u.mulations of chalky or earthy limestone in the Paris basin, and the beds of so-called Greensand, and Chalk of the same age, so largely developed in England.