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 shore. 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, constituted the
fresh-water fauna of the period.
The terrestrial flora of the period consisted of Ferns, Cycadeaceæ, and
Conifers; in the ponds and swamps some Zosteræ. The _Zosteræ_ are
monocotyledonous plants of the family of the Naïdaceæ, 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 masses of verdure appear somewhat exposed. They would
form a retreat for a great number of marine animals, and afford
nourishment to others.
* * * * *
[Illustration: 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 Jurassic 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 assumed in the
Jurassic 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 _Archæopteryx_, 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 shells 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 (_Gryphæa
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-Château, Auxerre, and Gray (Haute Saône).
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 analogous 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, analogous 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 accumulated: 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 constitute 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 passed 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 shells, were
overspread with fluviatile mud, which became dry land, and covered with
a forest, throughout a portion of space 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 shells. 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.
[Illustration: 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 constitute the Jurassic 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 shells of
_Cypris_, _Limnæa_, and some _Serpulæ_ 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 accumulation of _Ostrea distorta_,
resting on fresh-water strata full of _Cypris fasciculata_, _Planorbis_,
and _Limnæa_, 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_, _Limnæa_, 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 Jurassic 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 mass has the
form of the roe of certain fishes. The question naturally enough arises,
Whence this singular oolitic structure assumed by the components of
certain rocks? It is asserted that the grinding action of the sea acting
upon the precipitated limestone produces rounded forms analogous 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 stalactite-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
constituted oolites--that is to say, concentric layers, as in the
Jurassic 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 accumulated round the débris 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 Jurassic 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 circumstance, 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 Jurassic formation.
It has recently been announced that these rocks have been found in
India; they contribute largely to the formation of the main mass 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 constitutes 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 space.
A map of the country at the close of the Jurassic 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
Jurassic beds of Dorsetshire and France were connected by a tongue of
land running from Cherbourg to the Liassic beds of Dorsetshire, and that
Boulogne, still an island, was similarly connected with the Weald.
[Illustration: 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.