The formation which represents the Lower Oolite, and which in England
attains an average thickness of from 500 to 600 feet, forms a very
complex system of stratification, which includes the two formations,
_Bajocien_ and _Bathonian_, adopted by M. D’Orbigny and his followers.
The lowest beds of the _Inferior Oolite_ occur in Normandy, in the Lower
Alps (Basses-Alpes), in the neighbourhoods of Lyons and Neuchatel. They
are remarkable near Bayeux for the variety and beauty of their fossils:
the rocks are composed principally of limestones--yellowish-brown, or
red, charged with hydrated oxide of iron, often oolitic, and reposing on
calcareous sands. These deposits are surmounted by alternate layers of
clay and marl, blue or yellow--the well-known _Fuller’s Earth_, which is
so called from its use in the manufacture of woollen fabrics to extract
the grease from the wool. The second series of the Lower Oolite, which
attains a thickness of from 150 to 200 feet on the coast of Normandy,
and is well developed in the neighbourhood of Caen and in the Jura, has
been divided, in Britain, into four formations, in an ascending scale:--
1. The _Great_ or _Bath Oolite_, which consists principally of a very
characteristic, fine-grained, white, soft, and well-developed oolitic
limestone, at Bath, and also at Caen in Normandy. At the base of the
Great Oolite the Stonesfield beds occur, in which were found the bones
of the marsupial Mammals, to which we have already alluded; and along
with them bones of Reptiles, principally Pterodactyles, together with
some finely-preserved fossil plants, fruits, and insects.
2. _Bradford Clay_, which is a bluish marl, containing many fine
Encrinites (commonly called stone-lilies), but which had only a local
existence, appearing to be almost entirely confined to this formation.
“In this case, however,” says Lyell, “it appears that the solid upper
surface of the ‘Great Oolite’ had supported, for a time, a thick
submarine forest of these beautiful Zoophytes, until the clear and still
water was invaded with a current charged with mud, which threw down the
stone-lilies, and broke most of their stems short off near the point of
attachment. The stumps still remain in their original position.”[65] See
Fig. 1, PLATE XIX., p. 261. (Bradford, or Pear, Encrinite.)
[65] “Elements of Geology,” p. 399.
3. _Forest Marble_, which consists of an argillaceous shelly limestone,
abounding in marine fossils, and sandy and quartzose marls, is quarried
in the forest of Wichwood, in Wiltshire, and in the counties of Dorset,
Wilts, and Somerset.
4. The _Cornbrash_ (wheat-lands) consists of beds of rubbly
cream-coloured limestone, which forms a soil particularly favourable to
the cultivation of cereals; hence its name.[66]
[66] See Bristow in Descriptive Catalogue of Rocks, in _Mus. Pract.
Geol._, p. 134.
[Illustration: Fig. 118.--Meandrina Dædalæa.
_a_, entire figure, reduced; _b_, portion, natural size.
(Recent Coral.)]
The Lower Oolite ranges across the greater part of England, but “attains
its maximum development near Cheltenham, where it can be subdivided, at
least, into three parts. Passing north, the two lower divisions, each
more or less characterised by its own fossils, disappear, and the
Ragstone north-east of Cheltenham lies directly upon the Lias;
apparently as conformably as if it formed its true and immediate
successor, while at Dundry the equivalents of the upper freestones and
ragstones (the lower beds being absent) lie directly on the exceedingly
thin sands, which there overlie the Lower Lias. In Dorsetshire, on the
coast, the series is again perfect, though thin. Near Chipping Norton,
in Oxfordshire, the Inferior Oolite disappears altogether, and the Great
Oolite, having first overlapped the Fullers’ Earth, passes across the
Inferior Oolite, and in its turn seems to lie on the Upper Lias with a
regularity as perfect as if no formation in the neighbourhood came
between them. In Yorkshire the changed type of the Inferior Oolite, the
prevalence of sands, land-plants, and beds of coal, occur in such a
manner as to leave no doubt of the presence of terrestrial surfaces on
which the plants grew, and all these phenomena lead to the conclusion
that various and considerable oscillations of level took place in the
British area during the deposition of the strata, both of the Inferior
Oolite and of the formations which immediately succeed it.”[67]
[67] President’s Address, by Professor A. C. Ramsay. _Quart. Jour.
Geol. Soc._, 1864, vol. xx., p. 4.
The Inferior Oolite here alluded to is a thin bed of calcareous
freestone, resting on, and sometimes replaced by yellow sand, which
constitutes the passage-beds from the Liassic series. The Fullers’ Earth
clay lies between the limestones of the Inferior and Great Oolite, at
the base of which last lies the Stonesfield slate--a slightly oolitic,
shelly limestone, or flaggy and fissile sandstone, some six feet thick,
rich in organic remains, and ranging through Oxfordshire towards the
north-east, into Northamptonshire and Yorkshire. At Colley Weston, in
Northamptonshire, fossils of _Pecopteris polypodioides_ are found. In
the Great Oolite formation, near Bath, are many corals, among which the
_Eunomia radiata_ is very conspicuous. The fossil is not unlike the
existing brain-coral of the tropical seas (Fig. 118). The work of this
coral seems to have been suddenly stopped by “an invasion,” says Lyell,
“of argillaceous matter, which probably put a sudden stop to the growth
of Bradford Encrinites, and led to their preservation in marine
strata.”[68] The Cornbrash is, in general, a cream-coloured limestone,
about forty feet thick, in the south-west of England, and occupying a
considerable area in Dorsetshire and North Wilts, as at Cricklade,
Malmesbury, and Chippenham, in the latter county. _Terebratula obovata_
is its characteristic shell, and _Nucleolites clunicularis_, _Lima
gibbosa_, and _Avicula echinata_ occur constantly in great numbers.
Wherever it occurs the Cornbrash affords a rich and fertile soil, well
adapted for the growth of wheat, while the Forest Marble, as a soil, is
generally poor. The Cornbrash passes downwards into the Forest Marble,
and sometimes, as at Bradford, near Bath, is replaced by clay. This
clay, called the Bradford clay, is almost wholly confined to the county
of Wilts. _Terebratula decussata_ is one of the most characteristic
fossils, but the most common is the Apiocrinites or pear-shaped
encrinite, whose remains in this clay are so perfectly preserved that
the most minute articulations are often found in their natural
positions. PLATE XIX., p. 261 (Fig. 1), represents an adult attached by
a solid base to the rocky bottom on which it grew, whilst the smaller
individuals show the Encrinite in its young state--one with arms
expanded, the other with them closed. Ripple-marked slabs of fissile
Forest Marble are used as a roofing-slate, and may be traced over a
broad band of country in Wiltshire and Gloucestershire, separated from
each other by thin seams of clay, in which the undulating ridges of
the sand are preserved, and even the footmarks of small Crustaceans are
still visible.
[68] “Elements of Geology,” p. 400.
[Illustration: XVII.--Ideal Landscape of the Lower Oolite Period.]
On the opposite page (PLATE XVII.) is represented an ideal landscape of
the period of the Lower Oolite. On the shore are types of the vegetation
of the period. The _Zamites_, with large trunk covered with fan-like
leaves, resembled in form and bearing the existing Zamias of tropical
regions; a _Pterophyllum_, with its stem covered from base to summit
with its finely-cut feathery leaves; Conifers closely resembling our
Cypress, and an arborescent Fern. What distinguishes this sub-period
from that of the Lias is a group of magnificent trees, _Pandanus_,
remarkable for their aërial roots, their long leaves, and globular
fruit.
Upon one of the trees of this group the artist has placed the
_Phascolotherium_, not very unlike to our Opossum. It was amongst the
first of the Mammalia which appeared in the ancient world. The artist
has here enlarged the dimensions of the animal in order to show its
form. Let the reader reduce it in imagination one-sixth, for it was not
larger than an ordinary-sized cat.
A Crocodile and the fleshless skeleton of the Ichthyosaurus remind us
that Reptiles still occupied an important place in the animal creation.
A few Insects, especially Dragon-flies, fly about in the air. Ammonites
float on the surface of the waves, and the terrible Plesiosaurus, like a
gigantic swan, swims about in the sea. The circular reef of coral, the
work of ancient Polyps, foreshadows the atolls of the great ocean, for
it was during the Jurassic period that the Polyps of the ancient world
were most active in the production of coral-reefs and islets.