_a._ The so-called _Lower Green Sand_, named in contradistinction to the
_Upper Green Sand_, includes a series of iron stained sands, sandstones
and clays of great thickness. The clayey beds are seen at Atherfield in
the Isle of Wight, and at Nutfield in Surrey, while the sandy beds are
met with at Speeton, at Folkestone, and near Reigate. Besides
brachiopods and oysters, these beds have furnished a species of _Perna_
(_P. Mulleti_), an elongated mussel (_Gervillia anceps_), a pretty
_Trigonia_ (_T. cordata_), some _Ammonites_ and Nautili, with the teeth
and bones of big reptiles. The celebrated "Kentish Rag" and the sponge
gravels of Farringdon are of this age.
_b._ _Wealden._ The main mass of these rocks occupies the area inclosed
between the North and South Downs, and forms the Valley of the Weald,
whence they take their name. They consist of a series of sands,
sandstones, clays, and shelly limestones that were deposited in the
delta and off the mouth of a big river. The shells in them belong to
freshwater genera, _Cyrena_, _Unio_, _Paludina_, etc. Bones of a huge
lizard that hopped along on his hind legs (_Iguanodon_), and those of
crocodiles, etc., are from time to time brought to light. The Wealden
rocks occur also on both eastern and western sides of the Isle of Wight,
and in Dorsetshire.
[Illustration: _Inoceramus concentricus_ (from the Gault).]
3. Oolites (or Roe-stones) are so named because the characteristic
limestones of this formation resemble very much the roe of a fish. The
small round grains, of which the typical examples are built up, when cut
or broken through will be seen to be formed of numerous layers of
carbonate of lime, disposed like the coats of an onion, around some
central nucleus, generally a grain of sand, a fragment of coral, or the
shell of one of the Foraminifera. They are divided into Upper, Middle,
and Lower Oolites, and these again are subdivided as follows--
Upper Oolite.
_a._ _Purbeck Beds_, a series of fresh-water, with a few estuarine, or
marine beds, which in point of fact connect the deposits we are next
coming to with the Wealden just passed. They contain numerous
fresh-water shells--_Paludina_, _Physa_, _Limnaea_, etc., with the
microscopic valves of the little fresh-water crustacean _Cypris_, whose
descendants are abundant in the rivers and lakes of to-day. An oyster
occurs in the "Cinder Bed" and Plant remains in the "Dirt Beds." But the
Purbecks are best known for the numerous remains of small mammals
(_Plagiaulax_) allied to the kangaroo rat, at present living in
Australia.
_b._ The _Portland Stone and Sand_, which come next in order, are
largely quarried in the island whence they take their name. The
quarrymen point out fossils in the stone, which they call
"Horses'-heads" and "Portland screws." The former is the cast of a
_Trigonia_ shell; the latter, that of a tall spired univalve
(_Cerithium_).
In Wiltshire, a coral (_Isastrea oblonga_) is found in the sandy beds,
the original calcareous matter of which has been replaced by silex.
_c._ _Kimmeridge Clay._ This, by the pressure of the rocks subsequently
deposited on it, has in greater part been hardened, and possesses a
tendency to split in thin layers, and hence is termed by geologists a
shale. It is seen at various points between Kimmeridge on the
Dorsetshire coast and the Vale of Pickering in Yorkshire, and forms
broad valleys. The principal fossils in it are Ammonites, a
triangular-shaped oyster (_Ostrea deltoidea_), and one resembling a
comma (_Exogyra virgula_).
Middle Oolites.
_a._ The _Coral Rag_, or _Coralline Oolite_, comprises a most variable
set of beds, but principally a series of limestone, with fossil corals
still in the position in which they grew, and resembling in form the
reef-building corals of the Pacific. They rest on
_b._ _Oxford Clay_, a dark blue or slate-coloured clay without any
corals, but containing a great many _Ammonites_ and _Belemnites_. The
_Kelloway Rock_, a sandy limestone at the base of the Oxford Clay, is
well developed in Yorkshire, and furnishes amongst other fossils a large
belemnite and an oyster (_Gryphaea dilatata_).
Lower Oolites.
_a._ _Cornbrash_, a very shelly deposit of pale-coloured earthy, and
rubbly or sometimes compact limestone with plenty of fossils. The
commonest are Brachiopods, Limas, oysters (_Ostrea Marshii_),
Pholadomyas and Ammonites. It is best seen in Dorsetshire,
Somersetshire, and near Scarborough in Yorkshire.
_b._ _Forest Marble_ and _Bradford Clay_. The former is an exceedingly
shelly limestone, often splitting into thin slabs. On the surfaces of
some of the beds may be seen the ripple marks the sea made countless
years ago, and the tracks of worms and crabs that dwelt in the mud or
crawled on its surface at a time when it was soft mud. The Bradford clay
is a very local deposit, taking its name from Bradford in Wiltshire,
where it is most developed, and its characteristic fossil is the
pear-shaped Encrinite or "stone-lily" (_Apiocrinus Parkinsoni_).
_c._ The _Great_ or _Bath Oolite_, comprising a series of shelly
limestones and fine Oolites, or freestones. The latter are largely
quarried in the neighbourhood of Bath, and used for mantelpieces and the
stone facings of windows. The great Oolite is rich in univalve mollusca,
amongst which may be noted a limpet (_Patella rugosa_) and the handsome,
tall-spired _Nerinaea Voltzii_, numerous bivalves belonging to the genera
_Pholadomya Trigonia_, _Ostrea_ (_O. gregaria_), and _Pecten_, besides
Brachiopods (_Terebratula digona_, which looks very like a sack of
flour, and _T. perovalis_, etc.).
At the base of the Great Oolite are the "Stonesfield slates,"
so-called--a series of thin shelly Oolites, etc., that split readily
into very thin slabs. They are principally of interest to geologists on
account of the discovery in them of the remains of small insect-feeding
and possibly pouched mammals. With these are associated the bones of
that big reptile the _Megalosaurus_; the flying lizards called
Pterodactyles; fish teeth and spines; lamp shells; oysters, a _Trigonia_
(_T. impressa_); and the impressions of insects, including a butterfly,
and of plants.
_d._ _Fullers' Earth_, a clayey deposit occurring in the southwestern
parts of England, but not in the north. It abounds with a small oyster
(_O. acuminata_) and Brachiopods (e.g. _Terebratula ornithocephala_),
etc.
_e._ _Inferior Oolite_ (including the Midford Sands). As these beds are
followed across the country from the south-west of England to Yorkshire,
they are found to change greatly in character. Limestone and marine beds
in the south are replaced by sandy and estuarine beds in the north.
Amongst other fossils from beds of this age may be found several
Echinoderms, a crinkly lamp shell (_Terebratula frimbriata_), and a
spiny one (_Rhynchonella spinosa_), bivalves belonging to the Genera
_Ostrea_, _Trigonia_, _Pholadomya_, etc., and some very handsome
Ammonites (e.g. _A. Humphresianus_).
[Illustration: _Ichthyosaurus_, or Fish-lizard (from the Lias).]
[Illustration: _Plesiosaurus_ (from the Lias).]